What resolution is your monitor?
The 9900k is a solid cpu but the extra cost for it is your decision. Unfortunately this is a dead end platform, only downside. But the 9900k should last for many years to come. And the 9900k runs hot and requires a more expensive cooling option vs the 3700x.
The 3700x is a great all around cpu and ~$150 cheaper than the 9900k for similar performance especially as the monitor resolution increases.
IMO, the downside for Ryzen is the choice in AMD motherboards right now. The x570's are expensive due to the higher power requirements to power PCIe 4.0. You can get a 450-MAX series for a lot less money compared to the x570's. If you have the budget I'd suggest the x570 but the 450's are the best choice if cost is an issue.
Hopefully the 4000 series Ryzen cpu's will be able to run on the 450 and 470 motherboards but this has not yet been confirmed that I know of, someone correct me if I'm wrong please. So the 450's could be a dead end platform too.
The x570 will support the 4000 Ryzens so you'll have one more cpu upgrade available down the road. I think AMD is making a mistake having not yet released the 550 motherboards by now.
I built an x570 3800x pc last November and have absolutely no complaints. Everything has run without any issues and the gaming with a 2070S is very, very smooth at 1440p.
Good luck with your decision.
View: https://www.youtube.com/watch?v=EtIfYHI0faw