Is there a modern equivalent of that? Basically turn it into a thin client?

Well, X is still out there with its thin client capabilities intact. There are Wayland-compatible VNC clients and servers, if one isn’t big on X. SPICE is intended for connecting to VMs as servers. RDP if you want to use a Windows box as a server.

For a machine such as the OP describes, it would also be possible to install a tailored distro and software selection into the onboard space and place /home and such on a network drive, although that makes it impossible to take the tablet out of range of the LAN. If the touchscreen doesn’t work under either the Wacom or libinput drivers, it would probably be a waste of time, though.

(Really, 16GB is plenty for the distro itself—if I remove the three kernel source trees, a couple of games, and some FreePascal stuff, my desktop system minus /home would fit in that, and it’s anything but minimalistic.)

Yeah, that’s how I was taught in school in Canada in the 1980s, although no one ever explained why. It always did seem odd.

What’s the output of cat /proc/asound/cards on your system?

The 6.6.x kernel series is LTS and should be fine as a downgrade target (6.7.x not so much so). Unless there’s something specific from the newer kernel versions that you need to drive that system, there shouldn’t be any issues. I’m still on a 6.6-series kernel.

That being said, you could try troubleshooting this from the bottom up rather than the top down.

First, use lspci -v to verify that the device is being correctly identified and associated with a driver.

Next, invoke alsamixer and make sure everything is unmuted and your HD audio controller is the first sound device. The last time I had something like this happen to me, the issue turned out to be that the main soundcard slot was being hijacked by an HDMI audio output that I didn’t want and wasn’t using, and that was somehow muting the sound at the audio jack even when I tried to switch to it. A little mucking around in ALSA-level config files fixed everything.

Wild idea: check the condition of the SDD (presumably) that you’re trying to install to. After all, an installation has two endpoints, and if the target disc is on its last legs and throwing SMART errors, it ain’t gonna be too happy getting written to.