The cutting tool from prusaslicer works pretty well for this. For the aesthetics, I just try to align the cuts as natural as possible in the model.

I have been working on costume parts, my process with split pieces is first assemble with painter’s tape to get a test fit, disassemble, super glue the pieces together with locktite gel control, sand with 80 and then 220 grit, paint with rustoleum 2-in-1 sandable filler, fill in gaps and seams with bondo glazing spot putty, sand the entire thing with 220, and then repeat paint-bondo-sand until I am happy with the surface before I move onto actually painting.

Basically this process.

If I don’t want to paint something, and don’t care too much about layer lines, I’ll usually use super glue and then a wood burner to melt the edges together. It isn’t the prettiest, but it works fine.

Make the seams to print in one place in the slicer and grind the place off

I’m a big fan of welding parts together using a 3D pen and the filament I printed with. Then, if it’s not super clean, I make the edge flush with a small chisel.

That or uv adhesive to tack parts together and then CA glue and remove the adhesive.

I just bought a bigger printer. No more seams! Realistically though, plastic welding or epoxy and paint, and a whole lot of sanding.