OK, so update & happy customer here.
MTP is in fact slow.
But the real problem I had --- the media player quitting and restarting --- was due to a microSD formatted as exFAT, not FAT. I knew the difference, just pulled the card from my other device and popped it in expecting it to index the media library and just work. Oops! Jose at Moon Audio set me straight with a couple of quick emails, and what took me so long in posting back here was the delay in getting stuff on the card. A&K answered too, with the same info. Once I got the card loaded up and in the unit, it took a while to index, which people had warned me about and that's OK.
So some general comments on the unit now that I have it working.
0. The build quality is good. Really, really good. It has a nice heft in the hand, and it's a good size. I'm not about to drop it (if I can help it) or pound nails with it, but it definitely feels like it would survive both. It's something I'm happy just picking up and putting down.
1. Once the card is indexed the media player navigation is fine. Fast scrolling, good browsing, reasonable time loading album art. I don't feel like I'm waiting for anything when I go to pick my music. I like the navigation metaphors now that I'm finding my away around. I'm still a little slow getting to the top bar where you chose Artist/Album/Genre/whatever --- from a given list you click the v it's slow to load the menu. I'm not sure if the hit area is too small, or if it's doing something, or both.
2. I like the visual theme skin / red look. It's not a reason to pay this much for a DAP, but liking it Is good.
3. I've not done a whole bunch of listening yet, but so far it sounds great. Every bit as good as I hoped. I'm hoping to do some comparisons with the old Sony NW-A57 when I have time in the next few days.
4. Ditto on battery life --- so far it seems fine.
5. Having Tidal, Spotify, and Qobuz is definitely a nice step up from non-Android DAPs. I am looking forward to using that around the house.
Kudos to Jose at Moon Audio and the A&K folks for getting back to me so promptly, and I'm quite happy that my issues were me being an idiot and not anything else. When I'm thinking about headphones, I'll definitely give the Moon Audio folks a shot at my business, and A&K has ensured another happy customer.