Use the old lead containing solder; the new lead free stuff isn’t as fluid and doesn’t it wet the surfaces as well. Also make sure your torch has a large enough flame. I use a prestolite air/acetylene when doing copper typing. Also no matter how clean it looks don’t skip the sanding and steel wool step and don’t skimp on the flux.
Or alternatively if you have oxy-acetylene you can braze it that way you can just build a fillet around the outside of the joint, but I find brazing a lot harder to get a nice looking result - probably because I only do it rarely.
Oh, I have lots of experience with the lead free solders vs. tin/lead! In fact, right here I’m using a nice big spool of 63/37 (eutectic) solder I “lucked in to” on eBay a few years ago.
Brazing would be a problem here - too much heat for a nearby part. And, yeah, hard to get it to look good. I had a neighbor (deceased now) who was pretty good at it.