There are the freebsd mailing lists and worst case, file bug reports. Sometimes it takes a little while to get any kind of response, but if you provide as much detail as you can, it helps in the long run. RX6400, that's AMD? I think AMD is that gray area where "should be fully supported but sometimes newer generations are behind" (that is my personal opinion and pure conjecture).All I know is my RX 6400 doesn't work and if there's anyway I can help make that dream a reality I will.
As for AMD stuff, you may want to take a look at DragonFlyBSD. It was forked from FreeBSD around 4.x/5.0 and Matt seems to have found/tweaked alot for AMD. It could be useful as a datapoint.
Just want to note, above is all my personal opinion, I'm just a long time user of FreeBSD, not officially anything, except maybe "that guy that's older than dirt"