OK, I dug deep & found the solution.
Don’t really know how to explain, so I’ll use simple words. I did a lot of Google searches & carefully went through all outputs related to printers in the Terminal.
captstatusui -P LBP3000
was giving me a “Communication Error”. Printer was “installed as an USB device”, in CUPS the connection was
usb://Canon/LBP3000?serial=0000A18108DW
I changed that to
ccp://localhost:59787
Did
sudo systemctl restart ccpd.service
And it’s working now!
captstatusui -P LBP3000
also reports “Ready to Print”.
Hope it helps someone else. Best regards and thank you all for all the efforts!