DDR5 is (still) too expensive for me personally, and the lack of choice of 32GB modules and the limitation to 4 RAM slots on the boards make it impossible for me to upgrade anyway - nevertheless, my mouth waters every time I read the beautiful sequence of numbers 12900K - laugh - the predecessor runs at the limit for 10-15 hours a day.

In my opinion, this question makes no sense. Either your mainboard supports DDR4 or already DDR5. DDR5 is certainly faster, but also more expensive and only a few CPUs support it so far. There are enough articles on the net comparing DDR4 and DDR5 RAM.

Personally, I would not go for DDR5 yet, as it is still too expensive and not widespread enough.