What Size/Resolution External Monitor to Use with MacBook to Avoid Tiny Fonts?

Before you return this display launch System Preferences and select Displays. A dialog box will open on the display and another on your computer. In the dialog on the external display you'll see a checkmark labeled scaled. Click on that checkmark and you'll see several options that are good for your display. I'm sitting at my work computer (one display) so I'm working from memory. I think the options will be listed in terms of font size but it might be resolutions. Click on the various options to see if you find one that works for you.

My 28" 4K display at its default resolution has uncomfortably small fonts but the next rez down is perfect.

Make sure you have turned OFF Mirror displays in one of those panes, so that the external display and built-in display can be uncoupled.

Try holding the Option key while you click on the Scaled or resolutions box to see if any more are available.

You said you have been fiddling making fonts larger. Did you undo those changes, now that all are larger? or have they now added together?

I mostly use my LG display with a 2018 Mac mini and I've used both DisplayPort and HDMI - I get the same resolutions using either set of ports. I originally used an HDMI cable because it came with my display but late beta of Catalina had sync problems with HDMI so I switched to DisplayPort for a while. Presently my mini is connected with HDMI and I sometimes connect my MBAir to it via displayPort. I get the same resolution options on the Air as I do the mini.

Your display has fewer native resolutions than mine. I have 5 which run from 1920x1080 to 3840x2160 - I usually use it on the 2nd lowest rez which is 2560x1440.