There is actually an easy fix for this, as it turns out.
Abilitynet.org has a nice clear description at https://mcmw.abilitynet.org.uk/apple-os-x-10-10-yosemite-making-text-larger/
I didn't discover that source until after I'd called Apple Support, where I was given the same very simple answer, which is:
On the Apple Menu, go to System Preferences. In System Preferences, go to Displays. Change from Default to Scaled, then click the box to the left of the Default box to choose larger font or gigantic font. (Unfortunately, the Scaled display options are not available with a slider, which would have been perfect as then you could choose exactly the size that works best for you. However, the next size up from Default will probably work well for almost anyone who's been struggling to read the menu bar.)
Making this change not only fixes the visibility of the menu bar, but also carries over to other application menus. For example, I use WORD, and was having to peer at my 27-inch screen from 2 inches away to see where to navigate the cursor in order to change the font size for my document. In Google, if I wanted to read an url, I had to copy and paste it into WORD, then zoom the text display. It was VERY frustrating, and now all of that's just a bad dream.
((As an aside, as a "newbie" to the Apple world, this was my first contact with Apple Support and they earned top marks from me for response time, courtesy, and giving me precisely the info I needed. Thank you Apple Support!)