Thank you very much – but hopefully your kind offer will not be necessary
since it seems as if I have managed to circumvent the problem (after having
spent a ream of paper sheets and six oceans of ink drops).
It turned out that my problem was due to (at least) two oddities:
Every time I (re)started CUPS, it automagically changed the driver from what I had picked
to a driverless version (!).
The printer was set for WiFi/Network printing.
I turned off WiFi/Network printing and connected the printer to the Linux box via a USB cable.
Then I just had to add the “new” printer into CUPS, where I could select the proper driver.
So far, everything seems to work just fine. I can even print from The Gimp 2.9.9.
After this detour, I can go back to my luvvely profile tweaking experiments
(spending new reams of paper and new oceans of ink drops) again…