Mistwell
Crusty Old Meatwad
I am betting your fellow players prefer the careful fear/hyptnotic pattern after just a few times trying out the polymorph?Half cover is always situational, PCs can move around the T Rex.
Yes Im playing a Shadow Sorcerer level 9 , Twin Polymorph + The Hound Works Very well.
Or
Careful Fear/Hypnotic Pattern + The Hound is too scary
You might try using the Hound plus a bonus action Mind Sliver followed by a polymorph on the foe to turn them into a slug? That takes them out of the battle so you can deal with other foes first.