Anything that is one part, relatively thin, and dries opaque? Glossy is nice, but I don't want to see layer lines. Preferably something that can be painted over. I use enamel paint usually.

Thin and opaque? probably not.

auto body glazing putty is probably going to be your best option for what you want. not thin by any means - it is more of a paste, but is one part, opaque (usually a reddish color), and can easily be painted over.

glazing putty is not the same as body filler, either.. glazing putty comes in a tube. you just smear it on to fill in small holes/imperfections, or in this case, layer lines. once dry it is easily sanded and painted over.

Testers also makes a glue that drys clear as glass but goes on milky it comes in a wedge shaped plastic bottle with long thin neck at hobby shops, it could be filled in on that Dragon eye over black painted area and when dry would be glossy like the rest with black inside for example. I use it to make tiny windows on models and such, its sold for clear plastic part cement that wont fog windshields like normal plastic model glues and does as claimed. costly compaired to other glues though and worth it.

i have also used that glue - and it is great for flat things, but it would have been far too runny for use on the eye which is a round object..
the 2 part epoxy is thicker so it stayed where I put it, but smoothed itself out in that area. it did exactly what I wanted it to do.

Thank you!

I am quite pleased with how the whole thing is coming out at 300% scale - the ball joints have been a little bit of a challenge with the scaling, but it is getting there.

there is a product out there meant for coating 3d printed projects - i forget the exact name of it tho.. never used it.

others have used automotive body filler to smooth things.. usually larger items like cosplay costumes & masks, but it could work for other things.

for some small objects, I have used a simple 2 part, 5 minute epoxy to smooth & give a "glassy" look.. like the eyes for the dragon model I am working on