At least on the AMD front the 6500XT will apparently be limited to short dual slot designs. RX6400 might be doable in a dual slot half height form factor (depending on how far below 75W it goes) but won't be readily available to consumers.
https://twitter.com/RyanSmithAT/status/1479210583148748801
https://twitter.com/RyanSmithAT/status/1479210583148748801
And I've received an answer from AMD: no sub-75W RX 6500 XT parts. 107W is the minimum TBP for that product. There will be sub-75W RX 6400s, but those are OEM-only. At best, you may be able to find one in the grey market.