That’s a neat looking trick, will have to see how well it works in practice. I can’t see it working too well if you need to feed the wire into a through hole.

My trick is to tin the joint and the wire, apply no-clean flux to either the joint or the wire (situation dependent), then bring the two together while reflowing the joint. Quite handy for soldering tight multi-pin connectors.

The real tip is buy flux, it’s just as important as solder.