Tl;dr: Bringing Intelligence up to 20 is necessary at some point, but doesn't have to be immediately. War Caster and Resilient are both great options and will help you maintain concentration. Heavy Armour Master and Lucky are also great options but won't have much impact on concentration. None of the other feats seem particularly worthwhile for you. Personally, I'd probably take War Caster and then decide between Heavy Armour Master and bringing my Int up to 20.

I'll address your points one by one, starting with:

Bringing Intelligence up to 20

You definitely want to bring your Int to max at some point, because it controls your primary means of attack. The question is whether you want to get it now, or leave it till later. With that in mind, consider what 2 more points of Int actually provide:

• +1 to hit with your spell attack
• +1 to DCs for your spell saves
• +1 to Int saves
• +1 to Int checks
• +1 to number of spells you can prepare

None of these are huge benefits in the short-term. Long-term, this makes you 5% more effective at most of the things you specialise in. (Warning: gross over-generalisation.)

War Caster

You don't intend to have both hands full, so this only gives you advantage on Con saving throws to maintain concentration, and spells for opportunity attacks. However, both of these are strong benefits. Advantage to maintain concentration needs no explanation, but spell opportunity attacks might not be as clear.

To start with, you're planning to be in the thick of the melee, so you should be getting opportunity attacks often enough to want to use them. Unfortunately, you aren't planning on using a weapon, and your signature spell Vampiric Touch won't work with opportunity attacks. (It specifically requires an action on your turn to make an attack with it.) So in your present state, you have no use for opportunity attacks. Giving yourself an option could pay off.

On the other hand, opportunity attacks aren't the only reaction, and you might want to save your reaction for spells like Shield or Counterspell. That said, these require resources, whereas with War Caster you can always use your opportunity attack to cast a cantrip.

Heavy Armour Master

As you say, the Strength is negligible where you're concerned, so the question is the damage reduction. This is a great option for someone planning to be in melee - 3 damage off every non-magical weapon attack adds up to be a lot of damage very quickly.

For concentration, it's a bit weird. It won't affect the vast majority of Con saving throws you make to maintain concentration, since the damage has to be at least 22 for it to have an impact. (Many high-damage attacks are magical, and it won't affect those either.) When it does affect the DC, it will only reduce it by 1-2. So as an option to help you maintain concentration it's a bit of a bust. It's still a great defensive option though.

Keen Mind

I can't say any of these features have ever come up in a game I've played. They're all highly situational and I can't find anything to redeem this feat other than "it's kind of cool, I guess".

Linguist

3 languages is pretty good, but there are spells that take care of this sort of thing for you. I've never seen a situation in game where a cipher would've been handy. If you're looking for a way to send messages to someone that no one can intercept, there are a variety of spells to achieve that.

Lucky

3 rerolls a day that you can choose to spend once you've seen the result? This is huge. You can spend them on an attacker's roll against you? That is just nuts. If you get critted, you can just wipe it away. Note that its benefits aren't nearly as potent on rolls that you make with advantage, but they're even more helpful on rolls you make with disadvantage. As far as concentration is concerned, you can fix a failed concentration roll 3 times a day, but you're likely to be making a lot of them, so something that isn't a scare resource and benefits all of them is probably worth getting.

Observant

This feat is frighteningly good. Just not for you. For a character who's maxing Wisdom and has proficiency in Perception, this makes them a portable radar station. (One of my players, the cleric, took Observant and has been making my life hell spotting every stealthed enemy and trap a mile away.) For you, it will make you almost as good as they were without it. If your party lacks a "sensor" character, it could be worth considering. Assuming you have one, I don't recommend this.

Everything in the above paragraph is based on Perception, without considering Investigation. That's because Perception is used far more often than Investigation, and passive Perception is used almost constantly whereas passive Investigation rarely sees the light of day. This can vary from campaign to campaign, obviously, but in general Perception is better.

Lip-reading is a cool little feature that I have seen used several times in intrigue situations, but I don't think it's worth spending a feat on.

Resilient

For maintaining concentration, this is pretty strong. More reliable than the advantage provided by War Caster, and scales better as you level. As well as that, though, there are a lot of effects that trigger Con saving throws. Most diseases, poisons, many spells, and a lot of monster effects. There's a saving throw for every stat, but Con, Dex, and Wis are still the most common, so this is definitely worth your consideration.

Ritual Caster

First and foremost, you have every benefit of Ritual Caster (as far as spells on the Wizard list are concerned.) Second, the benefits of this particular feat are highly dependent on what scrolls the DM feels like tossing your way. So the only quantifiable benefit of this spell for you is learning 2 1st-level spells from another class's spell list. Unless you can see something you really want (I can't), I can't recommend this.

Besides that, most rituals are either available to the Wizard or can be effectively duplicated by the Wizard. The only one I can see that you can't produce for yourself (barring a Wish, obviously), is Silence, which I can't quite see how you would use as a ritual. (It's more of a combat spell, and the 10-minute casting time is inconvenient in combat.)

So the question here is:

Do bonuses to 'your speed' apply to flying speed?

Sadly, the answer is that is that it's not clear. I'll break down the arguments for and against.

For

• The 'Using Different Speeds' section on page 190 of the PHB says that "If you have more than one speed, such as your walking speed and a flying speed, you can switch back and forth between your speeds during your move.
• The 'Flying Movement' section on the same page says "If a flying creature is knocked prone, has its speed reduced to 0 [...] the creature falls [..]" which produces nonsensical results if 'its speed' refers to anything other than its flying speed.
• Unlike almost every other speed booster in the game, the Boots of Speed and the Boots of Striding and Springing both talk specifically about your walking speed.
• Crawford says that it does in one case, and in general.

Against

• Every other speed booster in the game just talks about 'your speed'. Some of them, like the Longstrider spell, are clearly only meant to be for walking speed.
• Mearls thinks bonuses to your speed only apply to walking speed. That said, he doesn't sound very sure, and he's not an official source of rules interpretation, so take that one with a grain of salt.
• Under the Speed section on page 182 of the PHB, it defines speed as: "Every character and monster has a speed, which is the distance in feet that the character or monster can walk in 1 round."
• Under the 'Your Turn' section on page 189 of the PHB, it includes this statement: "Your speed - sometimes called your walking speed - is noted on your character sheet."

To me, the last two arguments are the strongest ones, since they're the only attempts to actually define what 'your speed' means. That said, it's still a bit hazy, so I guess the answer (as ever is) has to be:

Talk to your DM. It's up to them to decide whether this works or not. I would definitely point out to your DM that this boost of 10 feet is effectively all you'd be getting from the Mobile feat, because:

• The second benefit of Mobile is pointless to you, since you're going to be flying all the time anyway.
• The third benefit of Mobile is equally pointless to you, since you're a flying Sorcerer. There's very little reason you'd ever get into melee, and even less reason for you to make melee attacks. (Although this works with melee spell attacks, so you can, for example, Vampiric Touch and then fly away without provoking opportunity attacks.)

Feats are meant to be powerful, and if all you're going to be getting is an increase from 60ft -> 70ft, it's not really a big deal. In fact, if I were you I probably wouldn't take Mobile. Haste is trickier, because it's a very small investment that would boost your speed up to 180ft (with the bonus Dash action), making you faster than any creature in the Monster Manual. Of course, it doesn't make much sense to allow one and not the other.