How do you handle joining new yarn in the middle of a project? My joins keep showing up as bumps

Been working on nalbinding for about six months now, mostly using wool that I've spun myself or bought as roving and prepped at home. The stitch is starting to feel natural, Oslo mostly, and I've gotten comfortable enough that I can do a few rounds without stopping to check a diagram. The part that keeps tripping me up is joining new lengths of yarn.

The felted join is what everyone seems to recommend, and I get why. When it works, it genuinely disappears into the fabric. But I'm having trouble getting consistent results. About half my joins felt together cleanly and the other half end up as a little lump that I can feel even if I can't always see it. My guesses so far: inconsistent fiber prep, not enough moisture, or rubbing too aggressively in the wrong direction. Probably some combination of all three.

A few things I'm curious about from people with more experience:

• Does fiber type make a huge difference here? I've been using a fairly coarse Corriedale and wondering if a finer or longer-staple wool would felt more reliably.
• How long do your overlapping joins actually need to be? Mine are probably around three to four centimeters and I'm not sure if that's too short.
• Is there a way to test the join before you commit and keep stitching, or do you just find out later?

Also open to hearing if anyone uses a different joining method entirely. I know felted is the traditional approach but I'm not precious about it if something else gives cleaner results.