I tried my hand at adding categories for the yaoi 添い遂げて、竜人さん, but realized that I'm unsure about the exact definitions of uke/seme. Doesn't help that this manga is bit of an odd one. Set in modern day with fantasy elements, the human protagonist lives alone, ends up giving shelter to a talking dragon. Interspecies sex ensues.

I have no idea which uke/seme tags to add, or which character even counts as which in this. The human is mostly of the serious type (borderline kuudere), the dragon is of the friendly goofball type.

The first sex scene is initiated by the pent-up dragon when they end up sharing futon, with the human as a somewhat reluctant bottom. And it's borderline sleep sex, with the dragon not remembering it well in the morning. In the other scenes the human protagonist is dominant and initiatives the sex (but is still on receiving end), whereas the dragon is mostly submissive.

Any advice on how to categorize that?

Also, should a talking dragon × human pairing be categorized as bestiality? The dragon can shapeshift into dragon-humanoid form, but he's in feral dragon form in two out of the three sex scenes in the first volume.

I haven’t read the manga, but the relationship will definitely be considered Bestiality from what I’ve read in other manga or DJs similar to this.
For the dragon talking and being a semi human sometimes, you can add the Anthropomorphism tag.
Since the uke, is more often in control and the dominant one, you can try the Dominant uke tag.
For the seme you can add as many defining tags you want. You don’t have to limit yourself to one tag per person. There can be multiple ones describing their personalities.
Side note: there’s a Talking animal/s category as well but idk if a dragon is considered an animal lol, more like a mythical creature.

Thanks, that helps a lot.
I'd think that since [u]Bestiality[/i] is applicable for the dragon, then Talking Animal is probably fine too.