Do you think font-size is a tad too small on HNs desktop version?

Yes, I have it zoomed in 3x to make it much easier to read. But it does handle zooming well and the browser remembers the setting, so it doesn't cause any problems for me.

I fondly remember the 90s, when 8pt was a normal font size, and 12pt was considered "big".

Folks have their system / browser font settings default to way too small now, but that's partially hardware folks' fault (people don't know their display is hdpi unless it renders super tiny text).

Maybe a font size knob could go into /profile alongside topcolor, but browser zoom works Really Well with HN on Desktop (150% - 300% works well for me).

Not really, but given how text-based HN is also scales nicely with browser zoom if you need it.