@Bob Jewett - are you suggesting I not worry as much about correcting the stroke and just practicing more? That's certainly a thought I've had, b/c every day after I run the Mighty-X drill I move on to other drills anyway, despite not having a straight follow-through, but I think it may still be worth tinkering with? Or maybe at some point I just live with it, but either way I'm definitely working towards the HAMB approach.
I run drills a good bit, haven't been as strict with requiring completion on those in awhile but I probably should. For example one of the shot drills I've used (TorX) requires has suggested requirements like making a shot 20x in a row. I'll admit to being lazy to those and stopping at 10+

That's why I mentioned missing easy shots out of nowhere - I have made those shots 10+ in a row with at least top, bottom, left and right english in shot drills, but sometimes in a game situation I miss it and I'm not fully confident it's not b/c of my stroke, you know what I mean?
How to measure progress? There's DrDave's BU RDS system with progressive levels and I've been doing that one recently. Other than that, I run various drills, like Niels Feijen's drills, where he suggests being able to run X in a row for C players, B players etc. so I strive to improve that #.