I wish this showed up in my feed sooner. I've stumbled into a lot of these fixes in my own, but i think this video would help a lot of people just starting out
I ended up eventually getting a Tedrow concertina which I like but generally it's just not worth buying anything cheap or Chinese made because the action is just poor. I think the price of concertinas is a bit inflated though, possibly due to Irish music competitions and the fact that good ones are made by just a few individuals globally. Best of luck to you!
Regarding the straps rubbing on your knuckles, if you make new ones you can shape them so the front edge steps in a bit in the wide section to give more knuckle clearance.
Oh, I totally forgot to mention that! Yes, the new straps should be reshaped but more importantly, you need to move the bridge/peak of the hand-rest lower. You can do this by making a new piece which is taller and remounting it lower or making a piece which attaches at the place (as shown here) but is shaped to bring the hadn't-rest down. For me, the new hand-rest sits just below where the old one was.
