Yesterday I replaced my system disk with a new one. Installed 20.04 fresh with partitions for swap, / , and /home. After that printers scanners seem to work better, however, HPLIP is not installed by default and I am not going to install it until/unless a new HPLIP release is made that explicitly supports 20.04. BTW, ippusbxd is not installed by default.
I think that at least some of my scanner problems stemmed from two things - (1) I have for some time done upgrades and not fresh installs; (2) At one point I tried to install HPLIP without carefully reading the release notes that indicate that 20.04 is not supported.
So, I have to accept some responsibility, but I think that it is surprising that HPLIP and 20.04 have not been made compatible. I hope that this happens.
Applications that scan properly are VueScan, scan2pdf, and "document scanner." Devices that work are the HP29W multifunction (usb scanning and wireless & usb printing), and an EPSON WF-4720 multifunction (wireless printing only), and an EPSON usb V370 scanner.
And, BTW, my first attempt at a fresh install of 20.04 failed! It encountered a known bug associated with 3rd party s/w installation during the process. I got around this by loading third party software (nvidia driver) after a stock install finished. This is the first time that I have seen such an installation failure.
I'm done with adventures for a while. Back to doing "real work."