I did that quite a while ago. Most of these need a 2nd look.
DC same as MM version, but I gave the big T. Con proficiency, which seems thematic to me. I think this is all good, to escape you need to either be really lucky or kill big T. That seems OK to me.
Limited magic immunity is the sames as Tiamat (also CR 30) from RoT. I just combined it with the big T's reflective carapace from the MM. The issue with the MM big T is that you could spam acid spells and take it down. This prevents that. It could probably be revised down some though. I will look at when I get to back to Monstrosities.
Yes, well that is the idea. I would need to check the encounter guidelines, but I am of the opinion that 4 lvl 20 PCs should not normally be able to take down a CR 30 monster.
990 HP/ 30HP/round regeneration and 21+4 damage per round each from 4 x 30th level Evoker Wizards dropping Meteor Swarm on the Big T. - who have all taken an extra 9 9th level spells and of course the 24 Int possible with the epic rules.
1st round: 920 HP after 4 Meteor Swarms
2nd round: 850 HP after 8 Meteor Swarms
3rd round: 780 HP after 12 Meteor Swarms
4th round: 710 HP after 16 Meteor Swarms
5th round: 640 HP after 20 Meteor Swarms
6th round: 570 HP after 24 Meteor Swarms
7th round: 500 HP after 28 Meteor Swarms
8th round: 430 HP after 32 Meteor Swarms
9th round: 360 HP after 36 Meteor Swarms
10th round: 290 HP after 40 Meteor Swarms
... and the cabal of the most powerful Wizards of four war-torn worlds seriously think about leaving in which round?
Of course the Big T. will fail to save against the odd one and they might just win actually... but this is assuming the Big T. stands there and lets them get that many rounds of spells off without killing them... I'm not sure CR 30 should look like this mechanically.
EDIT: of course, the damage bonus from being an Evokers gets reduced by the saves too - so it's even worse (redone)