EDIT: false alarm, it turns out - the solution below worked only the first time after reboot; once OpenCamera was backgrounded, and then opened again to take a new picture - upon the tap on the last photo taken thumbnail, I got the old stock gallery (with all its problems) again
Ok, here first I found:
There, there is a suggestion to
Open the settings app.
Go to “apps and notifications”.
There in the app listing tap the app in question, in this case the standard Gallery app.
Tap the “Open as standard” section.
Tap the “Flush standard setting” button.
Next time you do a preview from within the Camera app (or any other app calling for a picture viewer) you will be given the opportunity to select a standard app of your desire.
... however with the caveat "Dunno if I made the translation from my Swedish user interface correctly".
In any case, I tried to mess around with the above, and I experienced the same as in the response to that post:
On my phone (FP3), there is no standard settings linked to Gallery.
Then I found Open Camera / Discussion / General Discussion: Unable to set default gallery app:
One thing that finally worked: Settings -> Apps -> ... (The Three Dots) -> Reset app preferences.
After that, you will have exactly ONE CHANCE to choose the preferred photo application (as no app will be pre-chosen this time, all three should be equally available) in OpenCamera.
Also please note that "Reset app preferences" is a nuke - it destroys ALL your default app associations, all your blocked notifications, all background data restrictions etc. It's a pain in the ***, but nothing else ever worked for me :(
So, actually, I did that - and still, OpenCamera opened the last-taken-photo thumbnail in the stock gallery!
However, then I remembered this from the /e/ post, which I otherwise found cryptic:
Edit: after a reboot, everything is fine!
So, I decided to reboot the phone; opened up OpenCamera again, tapped the thumbnail of the last taken photo - and finally, I was asked to choose a Gallery app, and when I chose Simple Gallery Pro, I also got the option to choose it "Always", which I did... And finally, problem solved!
Just two weeks of fussing about for this. I can hardly wait for the next update to Android, where SD cards will be completely disabled