I think you need to be total CL 28 (therefore 2 non-caster levels are ok end-game wise). However you may get the final (or even final 2) Mythic Ranks very late, so maybe best not to set one's plans basing on those.
Well, you would know better than I if it's different in the main campaign. I'm telling you though that if you set up a CL28 multiclass in Inevitable Excess as a new character with the lich MP, it'll tell you you have 0 10th level spell slots when you cap out. Like unless that's buggy and it has to do with the specific way I got there (IIRC, it was a character that was like: Arcanist 6, Fighter 1, EK 10, Hellknight Signifier 3). I had level 10 spells, but no level 10 spell slots.
It's stuff like that that made me screw around with the DLC for theorycrafting purposes in the first place. I didn't want to have what I figured would be a solid build and then hit a wall in like hour 200.