Ok, after even more fiddling today, I believe I found the solution. At least… now it’s working again 
.
The issue seems all to be due to plymouth, that little piece of… software that runs right after GRUB, just before the login manager, that shows a splash screen with some moving thing in it… 
So, I got the hint because after trying another run of update with pamac:
❯ sudo pamac upgrade --force-refresh
[sudo] password for bill:
Preparing...
Synchronizing package databases...
Refreshing core.db...
Refreshing extra.db...
Refreshing multilib.db...
Refreshing core.files...
Refreshing extra.files...
Refreshing multilib.files...
Warning: plymouth: local (24.004.60-6) is newer than extra (22.02.122-16.3)
Resolving dependencies...
Checking inter-conflicts...
To upgrade (1):
thunderbird 115.8.0-1 (115.7.0-2) extra 58.0 MB
Total download size: 58.0 MB
Total installed size: -10.3 MB
Apply transaction ? [y/N] n
You see that warning that plymouth seems to have travelled ahead in space-time? 
Well, I’ve then simply run pamac again, allowing for downgrades, et voilá:
❯ sudo pamac upgrade --force-refresh --enable-downgrade
[sudo] password for bill:
Preparing...
Synchronizing package databases...
Refreshing core.db...
Refreshing extra.db...
Refreshing multilib.db...
Refreshing core.files...
Refreshing extra.files...
Refreshing multilib.files...
Warning: plymouth: downgrading from version 24.004.60-6 to version 22.02.122-16.3
Resolving dependencies...
Checking inter-conflicts...
To upgrade (1):
thunderbird 115.8.0-1 (115.7.0-2) extra 58.0 MB
To downgrade (1):
plymouth 22.02.122-16.3 (24.004.60-6) extra 1.0 MB
Total download size: 59.0 MB
Total installed size: -10.6 MB
Apply transaction ? [y/N] y
As simple as that, plymouth was restored to the current universe and everything worked again as if nothing has happened…
I’m not super amused at how this thing went down, to be honest… 