Exactly. the 7800X3D is currently the best gaming CPU but it's not enough of an upgrade for 5800X3D owners to bother with
If "Zen5%" applies to the 9800X3D too, then my 5800X3D will soldier on for another generation because nothing worthy has come along yet to make it look old and slow.
Not quite true, the 5800X3D is matched pretty closely by a Ryzen7 7700, which is a relatively cheap CPU.
There are plenty of affordable B650 motherboards. Over here £110 gets you an mATX board with VRM heatsinks capable of handling the 230W PPT of Ryzen 9 CPUs, and they're hugely overkill for the little "65W" 7700 which is probably best set to about a 115-125W PPT for the best balance of performance and ease of cooling.
DDR5-6000 EXPO kits are much, much cheaper than they used to be. Whilst it's true DDR4 is cheaper still, you do save money on the CPU with a 7700 so that neatly offsets the slightly more expensive RAM. You said "affordable, so we're only talking 16 or 32GB kits which is a $/£/€15-30 difference between DDR4-3600 and DDR5-6000.