You seem to have missed a critical part of sneak attack, and you seem to not understand the mechanics of grappling in much detail.
I'll deal with Sneak Attack first. I DM for a group that includes a rogue. He has been able to sneak attack about 95 % of his turns in combat so far, for one simple reason: the group has a tank that stands next to the enemies. The important part of Sneak Attack is as follows:
You don't need advantage on the attack roll if another enemy of the target is within 5 feet of it, that enemy isn't incapacitated, and you don't have disadvantage on the attack roll
If your group has a tank, they are most likely going to stand next to the enemies. Just make sure you're attacking the enemies next to your tank, and you sneak attacks are going to be available pretty much every turn.
Now, on to grappling.
Grappling can never activate sneak attack, since they are special melee attacks, not special melee weapon attacks. Grappling replaces one of your normal attacks when you initiate the grapple. Once you have grappled the enemy, you don't have to commit anything on your turn to keep the grapple going. There are two ways a grapple can end: If you intentionally let go of the target, or if they try to break free. The first option requires that you actually say that you let go of the enemy. The second option forces you to do an opposed athletics check, but on their turn, using their action. Once you have grappled your target, you can use your action on subsequent turns to attack them. If you have the grappler feat, or if an ally is within 5 feet of the enemy (likely the tank of your group, if you do a bit of team coordination), you will be able to sneak attack them.
Your build seems to be trying to solve a problem that, for the most part, doesn't exist if you play in a group. Simply coordinating with your tank so you're attacking enemies within 5 feet of them will activate your sneak attack every time, unless you have disadvantage. Rogues in 5e are balanced around being able to sneak attack pretty much every turn. If they can't, they fall behind on damage compared to even the most tank-focused of fighters.
It's already been pointed out that your question is perhaps a little too broad. So you may want to edit and narrow it down.
However, I'll focus my answer on a specific paragraph which seems to be your biggest concern:
I am the primary tank of the team so I need my AC high. In the adventures however, there seems to be so many opponents I soak up (usually 3 at a time engaging) that I am almost in low health before we win.
Tanking is not purely about AC. And if you obsess about this single aspect it will hamper your effectiveness.
An important aspect of tanking is control.
- When fighting unintelligent creatures, it may be easy enough to put yourself front and centre and they'll attack you. But more intelligent opponents may choose to ignore you and attack "better" targets instead. Champions tend to be focused more on raw power, so you'll have to rely on your teammates to cover this.
- But you might want to consider the Protection fighting style, which defends nearby allies by imposing disadvantage on an enemies attack roll.
- Sentinel is a good choice of feat.
- Mobile can be a useful feat making it safer to move around during combat.
AC suffers diminishing returns
Basically + 1 AC reduces average damage per attack by less for each point gained. The reasons are:
- Natural 20 is a guaranteed hit (+1 AC may in extreme cases may have zero effect of the range of rolls that succeed or miss).
- The critical effect boosts damage of a high roll (so AC increases reduce average damage / attack by less for each point).
Something that can be far more effective is imposing disadvantage or removing advantage.
At the extreme end, if an enemy needs to roll 20 to hit: +1 AC has zero effect on the odds of doing damage. But disadvantage reduces the odds from 1 in 20 to 1 in 400. And removing advantage reduces the odds from just under 1 in 10 to 1 in 20.
The benefit of AC is that it's passive. It's "always on" and available to avoid damage. It doesn't get "used up" and doesn't consume actions.
However, something seriously worth considering in place of a small AC boost is the Lucky feat.
AC doesn't help at all in some situations
Many magic attacks don't require an attack roll, meaning AC is useless. So you may want to consider alternative forms of damage avoidance.
- Shield Master is an excellent choice.
- Again the Lucky feat is noteworthy.
(Though I'm a little skeptical of the benefits of the Resilient feat.)
That said, unless you're in a very heavily combat and tactics focused group, you shouldn't need to min/max your abilities. In combat, teamwork and cooperation are far more important. And there's plenty that goes on in the game outside of combat.
Best Answer
No, that's not correct. Half-Orcs get the following ability:
As it says, when you get a crit, you get to roll one of the weapon's damage dice an extra time. So the first part of your calculation, the 3d8 damage your rapier will deal with a crit, is correct. However, Sneak Attack isn't part of the weapon's damage dice, it's extra damage that is added to the damage of the attack. Therefore, it's unaffected by Savage Attacks.
So your calculations should look like:
\begin{array}{c} \text{Level} & \text{Normal damage} & \text{Critical damage} \\ \hline 1 & 1d8 + mod + 1d6 & 3d8 + mod + 2d6 \\ 5 & 1d8 + mod + 3d6 & 3d8 + mod + 6d6 \\ \end{array}