Nice one! It's cool too see the progress you have made, I really think BletchMAME has huge potential to be a proper 'realtime interface' to MAME and not just another bloated dumb front end.
I think MAME has reached a point now, where it really needs a better way to get systems hooked up without getting lost and or relying on 'Mary's blog or some guy on reddit or some forums even here from 7 years ago etc etc'.
There are so many awesome old systems to tinker with that work good enough or are pretty much perfect and I wouldn't be the only one who finds it clunky and off-putting to workout how too connect anything up with a big long commandline, plus I worry about missing out on what's really available rather than default etc. A perfect example is, setting up a 486 or Mac etc. Basics such as Network, Sound Card (what works now, what's the best?) even a HDD from scratch, CDROM, MIDI, Serial etc + connecting things 'outside' the emulator box..
I really think half of the battle is won by having an a proper GUI that you can set things up and specifically with Bletch interact directly with MAME, I really think it it's the bridge lol. (you need a better name too, Bletch seems a bit awkward?).
Also, I don't know what it is and i'm interested in other opinions, but Qt always for me seems a bit laggy and it has these annoying interface things that make it not feel like a proper windows app. Just comparing your older version and the new one, I noticed searching and re-sizing windows are not as snappy. But other things are ok Qt fonts looks better now, I suppose more cross compatible now. But yeah Qt apps always seems a bit flickery/clunky with the interfaces, one of my gripes with QMC2 has something about it. But hey should be great to add a 'Dark theme' pretty quick haha.
Can't wait to see this progress!