Hello @yubiuser!
Thank you so much for caring. I really appreciate it.
I was a slightly bad person last night... Being a bit desperate, in parallel to posting my question here I also created a new post (where I also learned to include the pihole -d token URL!), and I got help already.
sudo PIHOLE_SKIP_OS_CHECK=true pihole -r
did the trick, since FTL for some reason hadn't been updated and I had a mix of v6 and v5.
I really appreciate you taking the time to try to help me, and I wish you a wonderful weekend ahead! 