if you have a PCIE 4.0 motherboard I would say 6400 all the way, it might be slower but is far better on power, and will be supported for a lot longer; unless you need more than 2 displays and a hardware video encoder;
I recently got into Polaris finally, but for me, I really didn't need more performance than my HD 7850 if I'm honest, but it was crashing in a newer game due to not being supported any more, so no hopes of fixing after a lot of tinkering... so, I found this cheap ex mining card, and there we go, seller looked very honest and sold another 30 of the same with no complaints (it's some clearly used XFX card, not some weird Chinese brand claiming to be new), so far so good, it's probably a card with at least 4 years of mining, but it's working very nicely atm.
oh and I'm still running Sandy Bridge with PCIE 2.0, so x4 from an rx6400 would be a complete disaster.
also I paid less than half of an RX 6400.
if you are feeling adventurous with Chinese cards, there are also some RX 5500XT for a very good price, so 6600, and even 3060s in some cases, those are far better than Polaris and probably supported for a lot longer.