FWIW I think I have a fix. It seems my laptop was going into s2idle by default. I don’t know why that causes a problem, but a bit of experimentation showed that ‘deep’ suspend works better for my particular model.
I made ‘deep’ persistent by adding a mem_sleep_default=deep kernel param (via putting it in a conf file in /etc/kernel/cmdline.d & then doing clr-boot-manager update).
So far so good.
[ tangent: oddly, this caused problems with my bluetooth mouse. The pairing via the gnome applet had been fine. Immediately after changing the suspend mode, it stopped reconnecting, even after a full reboot. Easy enough to fix by manually re-pairing with bluetoothctl, but odd nonetheless ]