Asus Tuf B450M-Plus Gamingor US$360
Asus ROG Crosshair VIII Hero X570
I'd suggest putting the $290 difference in the bank for now.
Thanks to both for your feedback. I think I'll go with a reasonably priced B450. The ROG B450 costs an extra $40 here ($140 vs $100, local prices) but has some extras that seem worth it: (a) Intel ethernet, (b) 2xM.2 (nice for a server) and (c) significantly better VRMs. There is also a much better ALC1220 codec for sound, which doesn't matter for me right now but may matter if at some point I connect the server to speakers...
In any case right now there is almost no availability for the 3900X, so I'll just keep waiting.