Can you use this ritual so you only need a casting tablet for a few minutes, and forever more count has having it in hand? Since the fact it's a ritual inherently makes it level 20, rather than the standard math of its effect, could one increase its target to Room (for magnitude 4 or 5, likely) and store an entire room's worth of books in loci with a single casting?
Remember a single locus is a short text, so you'd need an impressive Art of Memory Score to store everything.
As for the actual memorising a casting tablet, what separates a tablet from a spont. spell? If the only difference is the rigid methodology on the tablet, then memorisation will work. If tablets are actually magical, then it won't work (but will be able to recall it almost instantly, whatever that's worth)
There's nothing to stop you from inventing a higher level version that gives more or larger loci.
That said, I think for reasons of plausibility the caster should have to page through each and every text to be memorized, rather than performing the operation "in the exact time it takes to cast the spell" as the TMRE text states.
The level calculation is correct Base 5 +4 mags for complexity, the minimum level for ritualsdoeant apply in this instance.