I uploaded the first converted BG released at Winter C3 2021.
Here are the features. Most won't care and use the BG, but I decide to specify those, since I'm generous:
-can be 512x512: with some BG workaround, it can be - check;
-uses all layer 3 palettes: use SMB3 Stauts Bar.
-ExGFX amount: 3 + tilemap. This did cost me.
-No extra palettes: no, since some colors can be similar.
Update: Fixed sample level, since it lacked recommended layer 3 BG scrolling.
Credit can be optional, since I put a lot of effort here.