[RPG] How to make the final boss of the LMOP a memorable and exciting fight for the group

This post contains spoilers for the Lost Mines of Phandelver.

I have been DM'ing a group of six new players through the dnd 5e starter set and they have just entered the eponymous 'Lost Mine' where they will come face to face with their adversary 'The Black Spider' and a fight to the death will likely ensue.

My group consists of six players (a slightly large group), so I've already been in the habit of 'rebalancing' nearly every encounter that they come across, in order to maintain the appropriate level of challenge. I've largely used this online resource as a quick way to do so. It's normally suggested a combination of either raising HP, upgrading monsters to something thematically similar (ie – Ochre Jelly to Black Pudding), or adding additional monsters.

These three tactics are fine, for most normal encounters and I intend to employ at least two of them here, raising the BBEG's HP and adding a couple of extra minions. However, in addition to simply balancing the challenge level, because this guy is 'the final boss' of my players first ever DnD campaign, I want to ensure that this encounter is 'exciting and memorable'.

By 'exciting and memorable' I mean that I would like this encounter to feel appreciably different, perhaps surprisingly so, to all of the other, fairly vanilla, boss fights that have come before it in this campaign. So far they have all been tonally pretty similar to a normal fight, just a normal fight where the boss hits a bit harder and has more HP, meaning there might be a little more jeopardy. Note: Extra jeopardy is not bad, but that is not all, or even the main thing that I would like to achieve.

I would like this encounter to behave in ways that surprise my players 'mechanically', not just 'goodness, he hits hard'. I want this final encounter to be enjoyable, not just difficult. And, I still want to stay thematically close to the original intention of the villain (not just reskin a more powerful NPC from the MM, etc.). I'm not against player death, but I don't want a TPK at this stage in the campaign (and would fudge rolls / stats, if needed, to avoid one).

So, in summary my question is:

How can I make the final boss of the LMOP a memorable and exciting fight for my group?

General advice on boss building is not without value but has already been covered in a variety of other questions. Good answers here, would include reference to the specific parameters of this encounter, and might also include personal experience of running this module, covering things like:

1. What approaches have you used successfully in your own game to make the final encounter with The Black Spider 'memorable and exciting'?

2. What approaches have you used successfully in your own game to make the final encounter with The Black Spider feel balanced against a larger sized party?

3. Based on your own experience of this encounter, which of the following approaches, that I am already considering, would you recommend?

   1. Adding extra HP – (27 hp doesn't seem enough for a BBEG for a level 4/5 party of 6 PCs.
   2. Adding extra minions – (Accompanied by four Giant Spiders – might add two more)
   3. Adding Legendary Resistances
   4. Adding Legendary Actions
   5. Adding Lair Actions
   6. Turning The Black Spider into a "Paragon monster"
   7. Giving Giant Spider minions an 'on death' chance to use their ability Web (SRD p. 379).

Currently, I'm leaning towards a combination of approaches 1,2,6 and 7. My party = LMOP pre-gens (Fighter, Fighter, Rogue, Wizard, Cleric + an extra Vengance Paladin). We play using theatre of the mind, rather than a grid.