Bandcamp discontinued their API but you can still scrape their site, so long as you're polite about it. They rate limit pretty severely but it is possible. The individual pages now even have JSON data that has all relevant metadata.

All major search engines index Bandcamp, so I'm not sure they really can justify shutting down everyone else.

From what I remember, about 1% to 5% of artists or songs on bandcamp had libre/free (CC0, CC-BY, CC-BY-SA) terms. I would guess that the most likely reason it's so high compared with your number is that Bandcamp has a dropdown option, essentially, so they make it easy for bands to provide alternate licensing terms. Any libre/free licensing is going to be pretty niche and the people that care about such things is also niche, so I hear you that it's a lot of work for a very small payoff.

Consider the data set size, I was, at one point, considering just downloading all libre/free data, including songs, to avoid bit rot. Maybe I still will.

Doing a quick search with that data set I linked to, I see a couple dozen dub techno bands that are libre/free licensed.

Anyway, keep up the good work. I personally enjoy dub techno very much. I don't think I've ever met anyone offline who also likes it so it's nice to see other people who are passionate about it.