Hi I noticed on NeoForge 1.21 (DTN v1.18.28), when I try to make a BiomesOPlenty Wolf variant (the spectral one) into a DT dog, it resets to the Minecraft Pale variant instead of inheriting the BiomesOPlenty Spectral variant. Even tried manually editing the entity data but it just doesn't seem to even recognize it at all. Could, if possible, there be compatibility added in for modded wolf variants?

Currently unfortunately direct compatability for modded wolf variant of other party using vanilla wolf variant system is unsupported as DTN DogVariants system works a bit differently(as i think it is a better way for me to code on more special variants) and require specific mapping and registration for the variant to be registered, (in other word, it still possible to port the variant to DTN but it require the involvement of the other party) moreover, I dont think it is efficent to add direct and general support for other variant registering thru vanila system since its not an easy task and not a lot of mods have added wolves variants yet (only 1 as you point out which added only 1 variant) Maybe when there is more mod adding wolf variant we might reconsider.
However if you want more variant for DTN Dogs, we have another project called DoggyTalentsNext's Palette Of Paws which added a bunch of Wolf Variants which can exist on its own but is integrated with DTN as well.