That would have helped big-time. The thing is, I had to restore a TẂRP backup in order to get my old system back.
Now I am on 1.4q again, and the 400GB micro sd card works fine again - flawlessly. There seems to be new restrictions in Android 11 (and /e/1.5r) regarding storage space. And evidently a substantial bug in /e/os1.5r.
By the way, it took me lots of work and hours to restore the backup I’d made with TWRP. I ended up time and again with the dreaded “255” [extractTarFork( ) process ended with ERROR: 255] error. It took me tons of trials and errors until I eventually had success. What did the trick was: I wiped everything in TWRP, formatted the data partition, followed the official /e/ install instructions to the letter except closing the bootloader, booted the phone with Fastboot to TWRP and had TWRP flash the “data” partition of /e/1.4q only. I am writing this as there is evidently a lot of confusion about what to backup and what to restore with TWRP on which occasions.