I can't read what you have because I can't access google docs at work (a trying conundrum given the increased use of it on this board)

But even without reading it and just reading your post I have a suggestion.

If this "bound" class is supposed to replace all spell-casters leaving you with just a collection of mundane classes, why not just get rid of "spell casting" as a class entirely.

Keep all the mundane classes, then introduce the "binding" mechanic as an alternate system, like magic items. Any character can invest in it by making a pact with a spirit to get access to a spell or an enhancement or however you are coloring it. I assume there is a potential downside (the spirit demands something in return, that is more costly the more powerful it gets) So let that balance it.

Perhaps its purely financial (buy it) or time-consuming (downtime or in game time) or maybe they have to spend some other class feature (a feat? skill points? hit points? give up your level 4 attribute boost for a bind?) Maybe the "bound" is like a second class in addition to their primary class. Maybe you represent the bound as a new feature (you get one "bound" rank each level and you can spend bound ranks on spells. Or you can take your bound ranks and spend them for mundane features like extra feats, extra hit points, extra class skills, etc. if you don't want magic.

Some players may skip it entirely. Some get a few binds to complement their builds, some go hog-wild and end up more spell-caster than mundane but at a sizable cost.

But this way the player doesn't have to be a "bound" just to get the spell-casting and play with your new mechanic