Well, Z490/Z470 and 10900K/9900K would give you basically the same responsiveness in general usage, X299 was worth if you needed for heavy multi threaded workloads. Ryzen tosh seem to have issues with adobe software and some other hiccups here and there but it's doable, no one uses ryzen in this thread anyway. X299 is dead, and for that matter, intel is dead overall in MacOS world, putting you in a dead spot. I think building an hack now, specially an expensive one (and X299 boards and cpus are still expensive), is pointless. I would just wait for M1X, if you really need a heavy multithreaded machine and you need it now, then just buy an Asus board, Prime X299-A (II) and WS Sage (standard, 10G, II) are totally ok to my knowledge. Also keep in mind that you need at least a 14 cores part to see some gains to the consumer platform, 10940X 10960X and 10980XE are floating at the same prices at the moment, so just buy the cheapest you can find. You'll also need a GPU, and boy they are overpriced now.
I would buy an M1 mac mini and sell / swap it for M1X as soon as they release, you'll loose much less money this way and M1 performances are on par with 10 core Mac Pro and 10900X / 10920X ( at much much much less power usage)