I am HAPPY to report that I ran the steps, one by one, found in here
repair broken btrfs
… which was suggested by @awerlang
I also toggled to this webpage to compare the steps (SLES documentation):
How to recover from BTRFS errors
I am now booted in the the primary TW installation - root (BTRFS filesystem) is now mounted r-w, no more read-only mount ! The KDE Plasma login screen showed up as usual, I logged in and here I am 
I did run dmesg just after login to check the boot up results, and there are a couple of niggles:
[ 9.597221] BTRFS warning (device nvme1n1p3): checksum verify failed on logical 18335219712 mirror 1 wanted 0x29262f58 found 0x914d513d level 1
[ 9.597235] BTRFS warning (device nvme1n1p3): error accounting new delayed refs extent (err code: -5), quota inconsistent
...
... and ...
[ 32.462569] systemd-journald[701]: /var/log/journal/a9637f095381461d9ace3985c0ae5331/user-1000.journal: Journal file corrupted, rotating.
I did redirect output of the –repair, so we could see all the fixes, but unfortunately I wanted to append more info onto that file and I used the redirect > instead of append >>. Oh well.
I am going to boot out of here, to the secondary and run a check, then I’ll boot back to this primary and run a zypper dup.
So, a BIG THANKS to @awerlang and @arvidjaar and @dcurtisfra !!
.