Perhaps adding an override like what you did with optifine will make it work with sodium.

Isn't this mod already compatible with this version of Sodium?

It is still open on their side CaffeineMC/sodium-fabric#647. (Unless they forgot to close it?!?)

Maybe I'll test something, then edit this comment...

Maybe in config/sodium-mixin.properties

# previous properties
features.fast_biome_colors = false
# next properties

I finally got around to it. Added compatibility here:

0ac4ecc
4a6cdc8

At least now a compatible version exists until they decide to change their code.
It seems to work pretty well for now.

Is it possible for you to backport the compatibility to 1.16.5?