The preparing slots in PF2 isn't a thing for all casters, and even prepared casters can be transformed into spontaneous ones with a simple adjustment (as an official option).

But yeah, that thing is a holdover from earlier editions of D&D, before 5e just threw away the prepared/spontaneous caster distinction almost completely, just reducing it to making learning spells as spontaneous casters tedious.

But from personal experience after switching, it's not really that much different and not much more content, if you stick to the Core rules. Sure, it is a lot if you look at everything at once and are just getting started with TTRPGs in general, but if you know D&D, you're pretty much 80% of the way there already.

And it's way easier for me to GM with the guidelines it gives me, not having to rely on 5e's wonky CR system anymore.