Both Slide Over and Split View came with IOS 9, but only Slide Over worked with 1GB devices (and not even that with 512MB devices...)
Personally I don't use FCP, I use lumafusion (but I don't videoedit often), so FCP coming to iPad won't change anything for me (same for the other pro apps, no interest in Logic, I use Korg Gadget and Module)
I agree with you in general, but I would make a distinction. If you buy 128 or even 256GB iPads, it doesn't make sense to go 1TB for an amount of RAM that you won't fully use before many years if ever...
However if you go with 512GB then the jump is $400-500 and at that point it can start making sense, especially if you plan to keep the iPad for a very long time...
Personally I am one of those that always get the top specs (I got the maxed out version both with the 2015 pro and the 2018 11in pro). But now I am not in the market for a new iPad unless at some point iPadOS brings some very compelling, desktop-like, features that only work on M1...
But there is one thing that would make me upgrade instantly, without even thinking about it and not caring about price, but it won't happen anytime soon, a 15in iPad pro max...