This is a great post. I just recently created a lapping jig, like OPs. My issue is a bit different, it's an almost 50 year old barrel that was neglected by a previous owner, pitted, builds up crud and gets hard to load and clean faster than it should.
I started with brasso, and have gotten increasingly more aggressive with lapping pastes from 6k, I'm now at 220 grit wheeler lapping paste. So far I've been very carefully making sure the lapping compound is only on the high points, I don't want to grind the lands away.
I've spent about 1 hour and the lands are still well defined and the grooves are getting cleaned up. I still need a ton more work but it's promising.
There are obvious high spots and low spots in the grooves where it's polished one place but still very rough 12" away in the same groove.
Next time I'll post some before/after borescope pics that are blurry and pretty much worthless
