Raspberry Pi doesn't recognize HP PSC 750xi Scanner

I posted this to raspberrypi.stackexchange yesterday with no responses and only 4 views, so I'm reposting and expanding the original post here to reach a wider/different audience. I have an old Pi-3 running Stretch. I have what apt-get install tells me is the latest version of everything installed. I plug in an old HP PSC 750xi All-In-One USB printer/scanner. The Pi recognizes the printer features of the device, and I can successfully configure CUPS to use the printer. But I cannot get the Pi to recognize the device's scanner. I have a separate laptop running Ubuntu 20. When I plug the USB device into that laptop, I can use hplip's hp-scan tool to successfully scan. But none of the Pi's hplip or sane tools (hp-setup, hp-probe, hp-check, hp-scan, scanimage, sane-find-scanner ...) recognize the USB scanner. Clearly, as the Ubuntu test proves, hplip **does** support scanning for this all-in-one printer/scanner. And hplip documentation indicates that the device is supported both for the Pi hplip version (3.16.11) and for the Ubuntu version (3.20). I also have an HP 6978 Network printer/scanner that the Ubuntu system sees on the network and can manipulate with the hplip tools. The Pi's CUPS server sees and can use that network printer. But, like the USB device, the Pi fails to 'see' the network scanner. My understanding is that these scanners are supported through use of the libsane-hpaio backend. Yes, I have installed the libsane-hpaio package on the Pi. I've run scanimage on the Pi with SANE_DEBUG_DLL=128 and SANE_DEBUG_HPAIO=128 but the output simply reports no scanners found without indicating any errors of any kind. On the Ubuntu system, the same debug output shows successful discovery of both the network scanner and the USB-connected scanner. But there appear to be more fundamental issues. The full lsusb output is now reporting:

theadmin@RASSERVER:~ $ lsusb
Protocol spec without prior Class and Subclass spec at line 23291
Bus 001 Device 005: ID 0bc2:231a Seagate RSS LLC Expansion Portable
Bus 001 Device 006: ID 03f0:1511 HP, Inc PSC 750xi
Bus 001 Device 003: ID 0424:ec00 Microchip Technology, Inc. (formerly SMSC) SMSC9512/9514 Fast Ethernet Adapter
Bus 001 Device 002: ID 0424:9514 Microchip Technology, Inc. (formerly SMSC) SMC9514 Hub
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

It's that very first line I'm concerned about. I'm thinking this is a possible Python version issue as python -V reports Python 2.7.13. But Python3.5 is installed too (though I have no idea which package might have installed it,) and python3 -V reports Python 3.5.3. So, I'm thinking half the packages are using python v2 and half are using v3 and I don't know which is using which. Nor do I know how to configure them to use a different version. Plus, I'm a bit hesitant to even try to reconfigure/upgrade the base python from v2 to v3 for fear of breaking the entire system. And I'm concerned that the libusb and usbutil packages are so old, while hplip and its companions are much newer ... some kind of a mismatch somewhere, but I hardly know where to start. I hate to rebuild the Pi from scratch because I don't want to have to rebuild its LAMP stack, web server, nextcloud server, etc. but that may be my only option. I cannot find any missing packages or dependencies on the Pi. I have tried removing and re-installing hplip and a number of other packages. Clearly, there is some configuration difference between the Pi and the Ubuntu platforms. But after several days of struggle, I'm stumped. Anyone have any ideas?