It is a possible compromise to simply not add the more problematic spells. They have left the 1st lvl Conjure Familiar out, for example. Part of the problem is that such high tier spells tend to be very mechanically complex as well, which will be a strain for actually making them work. I'm not against it, but I'm not expecting it.
Something worth keping in mind is that the balance isn't just going to be a problem of full casters vs everyone else. At 13th level, half-casters and 3rd-casters also get a major powerbump, by unlocking 4th and 3rd level spells respectively. In Solasta, this is a major win for Paladins and Rangers. And as much as I enjoy Rangers (and love the new Swift Blade as a perfect callback to the dual wielding masters of earlier editions) Fighters are already having a hard time keeping up. If we're going to get more levels, non-casters are going to need really strong subclasses to compensate for it. Which I am in favor of, but Fighter has been pretty underwhelming until Commander.
Poor Fighter... they're almost as pointless as they were in 3rd edition. Luckily Commander has a fairly unique role.
This adds a lot to the discussion. Yeah, now I feel that not being able to have the non-spellcasting warriors keep up with casting characters is the likely reason they capped at level 12. Very unfortunate. Rather than all the gimmicks of insanely hard difficulty settingts, I feel a true challenge in Solasta is to get through the campaigns with a party of four fighters. :)
On Normal, that would probably work out eventually. It isn't like Fighters are horrible, because having all armor & weapons, Second Wind, Action Surge, Extra Attack 1 & 2 and bonus ASI can be useful. But Fighters are very lacking in their subclasses for everything except a dedicated Strength+Athletics sword'n'board tank in Mountaineer. Champion is awful and Commander, after having tried it for a bit, is honestly very underwhelming. Spellblades at least get some usable spells, like Fog Cloud and Misty Step. The option to pick up an extra feat is also worse in Solasta than tabletop because there are few feats as strong as Polearm Master, Great Weapon Master or Sharpshooter. The new Swift Blade complicated things even more.
Do I think Swift Blade Rangers are overpowered compared to the full width of 5e? Probably not, no. I'd rank them below the Gloom Stalker Ranger from Xanathar's Guide to Everything, but probably above all the other Ranger subclasses. They are, however, really strong in Solasta where competition is scarce, and they make it hard to justify using a Fighter, since they get both 3 attacks per turn and 3rd level spells. At least Path of the Claw Barbarians end up scaling really well into the late game if you want a warrior without spellslots.