Playing all classes on one character

My plan is to level all classes to 75 and utilize there class skill tree as if each of them were my main. So what does this mean? My sub class will be specced like a main class.

How gimped will that make me or will it only matter a little bit?

My backup plan is to spend $$$ to unlock a bunch of extra class skill trees.

It might not be too much of an issue, depending on how you build your Trees, especially if you primarily use Hunter as your subclass, since Fury Stance Hunter is one of the best subclasses for Fighter, Ranger, Gunner, Tecther, Braver, and Bouncer. You have to make some sacrifices in support Skills that you would normally have for a subclass for the sake of having Hunter weapon Focus Skills and Guard Skills, but it shouldn't be too bad.

From there it just leaves your subclasses for Hunter, Force, and Summoner. Fighter is a generally one of the best choices for all three, but you basically lose out on up to 15 SP just from Overload and Critical Strike since they only apply when Fighter is your main class, and even then the builds for Fighter for each of those classes varies because of their playstyle. Force also has the option of Tecther as a subclass, but a Techter sub Tree is usually built very differently from a main Tree.

@Isomer Extra skill trees only for a few classes will make sure that you're not gimped by the desire to become an allmighty allclasser. I think the advice given by @AndrlCh is quite spot on, only having extra trees for strong subclasses will give you the flexibility you are looking for. If you want to play all classes in endgame you might look into investing into extra mags, so you can have mags for every class (Ratk,Satk,Tatk & DEX), they "only" cost 300 AC each to get one.