Thumb shifters are definitely the way to go on a vintage flatbar conversion. The Sunrace M10 are a nice choice; I’ve used them to replace the ‘Gripshift’ on my kids’ bikes, they just work really well, especially for how inexpensive they are.
Another good bang-for-the buck upgrade is a modern SIS (Shimano or sunrace) freewheel and a modern 8-speed chain. Even with friction shifters and a vintage RD, the shifting is way smoother than with vintage ‘flat-cut’ stuff