First off, I love your model. That thing is utterly terrifying. Bravo!
@ad_hoc is totally right. This is too strong for an 8th-level party. Frankly, you took a somewhat comical CR 4 monster and you turned it into something rivaling the power of Jubilex itself! I strongly recommend you reduce this monster's power, both defensively and offensively, such that its CR is about 12 or 13. That should give you the tense dramatic fight feel you're aiming for without obliterating the PCs.
I'm going to run the numbers to evaluate its approximate Challenge Rating using DMG page 274, assuming you're removing the slashing damage immunity, and that your target CR is in the 11-16 range (a serious threat to your 8th level party, but not impossible)...
Defensive CR 24
Effective HP = ((312 + (3 * 20) regen )) * 1.25 resistances + (3 * 30) legendary resistances = 555
Effective AC = 14 + 2 saves + 2 magic resist = 18
555 effective HP puts it at defensive CR 24, and 18 effective AC is about right.
Note: I did not account for it using the Blur spell nor the damage-reducing aspect of Corrosive Form, which could conceivably boost its AC by maybe another 2-4, though accounting for that in the maths is more complicated than I had energy for. However, the damage resistance multiplier for a CR 24 should be negligible (i.e. x1), making its effective HP more like 462, which would make its defensive CR more like 22. With monster maths, you can really exhaust yourself trying to figure this out, so I just called these two competing factors a wash. If your 8th-level party is extremely well-equipped with magic weapons, then you could assume your current version of Garos is more like a total CR 20 creature. Still way overpowered.
Offensive CR 20
DPR = 5 lair action (green slime, single-round damage) + (2 * 36) two slams + 10 mind thrust + (2 * 14) two legendary actions: slams + (2 * 4) corrosive form = 123
Effective Attack = +10 +1 corrosive aspect of its slam reducing AC = +11
123 damage puts it at offensive CR 20, and its attack bonus is about right for that CR.
Total CR 22
Taking the average = (24 + 20) / 2 = 22
By way of comparison, Jubilex is CR 23.