The SD card mounts on boot after an initial warning that it can't, but after a few minutes the volume unmounts and the phone throws a system error complaining that the SD card is missing. There is apparently no fix except to reboot, but then the problem repeats. Removing and reinserting the SD card does not resolve the problem.
I might be able push off the problem for a while by buying a new SD card and seeing if that works any better. It might also be that the phone's card slot is unreliable, though, in which case that would not be a fix.
Because many Android app developers force their apps to reside in actual internal storage rather than adoptable storage--which should both in theory and by specification be just as good for app storage as physical internal storage--I was left with something on the order of 10GB of wasted space on the SD card anyway, even when it was working correctly. (I use my adoptable storage almost exclusively for app storage. At least I do for apps that allow me to, and at least when the adoptable storage is working.)
Now I am looking at new phones. The other major issue that has frustrated me about my phone is that like nearly all Android phones, OS updates are essentially mythical beyond a certain point. My vendor got as far as Marshmallow and that was it. (This is better than my backup Android phone, which never got any OS updates at all, but the phone was free.)
I think my next phone will have plenty of internal storage and no SD card slot at all. Also, it will come from a vendor which diligently pushes out OS updates.
My understanding is that these requirements restrict me to a choice between all of two phone vendors: Google and Apple. A Google Pixel with 128GB of storage is $749. An iPhone 7 with 256GB of storage is $849. Pricey.
Until I figure out what I'm doing about this problem reaching me by phone will be less reliable than usual. I do have my backup phone. If you'd like my number there please contact me. I also have a couple of Google Voice numbers which I should probably propagate as well.
Any suggestions as to how I should proceed and any thoughts on the pluses and minuses of my two options are welcome. Finally, do I have any other options, given my requirements?