I'm going to find out next week, stay tuned.
Seems like the biggest difference to me is that you don't get multiple attacks with the attack action. You do get those psychic d8s with your one attack, but that's about it. I'm hoping that the various utility aspects make up for the damage loss but we'll see; I'm mostly looking forward to using Knock Back, since Brute Strike requires betting the psi ahead of time rather than reacting to a hit. But of course you also have Bestial Form so you can spend 5 psi and a bonus action to fly, attune psionic weapon to get past DR, etc etc, which is all nice, and you can still spend just a single discipline choice to get proficiency with any skill for the cost of a bonus action.
Other big thing I see is that as an Immortal you don't get anything that lets you use your dex for weapons, which makes your Barb-esque bare AC thing less useful. You can get around it by going High Elf; you get +1 int and proficiency with the Shortsword. It's not a rapier but it is finesse, so then you can pile on the Dex/Con as needed without blowing a feat on Weapon Master. And there's the cantrip, ultimately I think High Elf's probably the best PHB race for an Immortal. You can also just use feats to get up to medium/heavy armor and focus on strength instead.