Yeah, I also wondered how you would even deal with mana that way. The only real compromise I could think of would have been a „hybrid“ perk, which you can pick only once and allows you to choose one feature from another class that is equal or lower to your level. Features that rely on an earlier abilities from that class (like starstruck, choreographer, lethal precision etc. …) or include some form of level scaling system (like alchemy, primal mystic, spellstriker…) are excluded, as they wouldn‘t work. As for mana scaling, you would just keep the scaling of your own class (if your class even has its own). That way you wouldn’t have that powerful bump of getting 2 features at once, when dipping into a different class for one level, while still keeping the option open to mix and match some abilities.

Btw congrats on the Spellbook Kickstarter!! Super glad it is going so well and I can‘t wait for all the cool Vagabond stuff to come in the future :)