I think you could do what you want by getting rid of Gnosis (which you are) and simply making Arcana into separate Merits. There's no real need, IMO, to make them linked to any one stat. Instead, if you want to limit their increase systematically, make sure that to have the next level, you need to already have a predetermined amount of Rotes in the previous ones.
For example:
Forces 1 requires no rotes (as there are no Forces 0 rotes; but it does open the character's knowledge of the Arcanum).
Forces 2 requires two Forces 1 rotes.
Forces 3 requires three Forces 1 rotes and two Forces 2 rotes.
Forces 4 requires four Forces 1 rotes, three Forces 2 rotes, and two Forces 3 rotes.
Forces 5 requires five Forces 1 rotes, four Forces 2 rotes, three Forces 3 rotes, and two Forces 4 rotes.
This is actually a system I have had in the backburner for a while and will probably use some day, but you're welcome to it if you like it, obviously. It makes characters have to broaden their horizons in lower levels before advancing--which is more "realistic", but more importantly, it makes sure someone who uses magic actually has an internal grimoire (so to speak) instead of cherry picking rotes they like and relying on creative thaumaturgy for everything else (which would be bad in your system anyway, since a lack of Gnosis would make their dice pools lame).