[Solved] OpenWrt on MT7621/MT7615N devices with 5GHz problems

Wish I had searched with more care; did not find this thread until after I bought an EA7500 v2 (cheaply on eBay) to use as an AP + managed switch. (Note this post does not seem come up in searches for "MT7615N" by itself. Possibly need to tag it or edit the title to put spaces around that term)

Although the performance is mostly very good, I've had to stop using it because of 5GHz connection stability; with my workflow the dealbreaking issue is that when connected at 5GHz, directory listings from an NFS server made from a Macbook Air Finder window just give me a spinner that goes on for an indeterminate time -- if no other network activity, forever. If I make the link busy with other activity (e.g. iperf3 between the same two machines) it recovers and I get the listing. I'll experiment with modifying the wifi settings as discussed here, but for now I'm back to using a slightly slower (and also quirky in other ways) ipq4018-based EA6350, identically configured which does the job well.

Edit: Problem occurs using OpenWrt 21.02.3 release.