I vote for cut the lines straight into your design with a small end-mill, then fill them with epoxy, ejected from a syringe, and re-machine the top flat after the epoxy cures. I did this in a bench project and also this entry way table project and it worked really well. https://forum.avidcnc.com/t/3d-sculpted-teak-epoxy-steel-entryway-table/1817/10 I have done this on ~5 projects now and it seems to work really well. I cut a little deeper than I think I need to and I use slow cure epoxy and give it the minimum 3x days to care.
This project speaks to my heart. A set of cribbage boards was the first project I helped my dad make for a set of Christmas gifts on one of my first home build small CNC machines with a 386 running hand coded software running in compiled MS-Quick Basic-Pro back in 1989~90. So many good memories from that project. Had to wrap the whole thing up in a black plastic trash bag and keep a shop vac running while cutting wood with the Dremel to keep it from slowly covering the room in sawdust.
Thanks for taking me on a trip down memory lane here. This is that first mill, below, and one of those first cribbage boards. I went back and milled very fine 1/16in slots in them but I don’t have any pictures of those. This one was the prototype which Dad let me keep. Dad had a company in town etch the brass plate with each of our names on them, but it doesn’t show in this ancient photo.
