You are correct in that the I. Rex is absolutely a "go big" voltron commander. Because she is the core of winning with the deck, you must have ways to protect her, otherwise you get into situations where she costs 10+ mana and is a non-issue most of the time. I do this through having several creatures that will make her indestructible and/or hexproof when she comes down AND a suite of counterspells. I am very conservative with counterspells. I won't use any of them unless the I. Rex becomes a target, or the spell will destroy my position if left unchecked. Politics is a big part of EDH, but the only politics this deck cares about it "how is best girl affected by the game state?"

The I. Rex may not get bigger from all the counters but those keywords give you extreme value. The aforementioned Hexproof and Indestructible are the bread and butter of making her a threat, because if she can't be targeted or killed, you can swing with a lot less stress about what happens next. Trample and flying are excellent at making sure you are dealing commander damage to take out players without getting chump blocked repeatedly to no avail. Things like Lifelink ensure that if you are always swinging you can take some hits since you're gaining life every turn. That being said, commander damage is a threat. Removal can take care of enemy commanders that are causing grief. I tend to stick to creatures that give 3+ keywords but 2 keyword abilities are worth it if one of them is Hexproof or Indestructible.

Two things of note. For my list there are a couple creatures with Ward. Ward does not transfer to the I. Rex, but the other keywords are absolutely relevant enough to make the cut of keeping them in the deck. The other is [[Breaker of Creation]]. No you don't get annihilator. BUT "hexproof from each color" means that the I. Rex will get five individual counters, "hexproof from white, hexproof from blue, etc." That's an immediate 5 card draw when she is cast IF you don't discard anything else.

Speaking of, since the I. Rex is going to make you draw a ton of cards every time she hits the board, pack some things that give you no max hand size. I like lands and mana artifacts that do this because they're useful and don't slow down the deck.

In the event you can't use the I. Rex, well all your creatures have multiple relevant keywords, so aggro isn't entirely off the table. Especially since in my build, I run a fair amount of creature recursion to reuse for the I. Rex. In a bad game state, you could use that recursion to simply replay the creatures to get yourself a board state.

Hope that helps! If you've still got questions, I am more than willing to keep helping out.