FOLLOW-UP: I had to focus on some other stuff for a while but got back to this the other day. I decided to pull the new SSD out and reformat it to HFS/MacOs Extended with my other Mac. I put it back into the laptop and this time it tried to boot from my High Sierra USB drive but it stopped partway thru with an error that it couldn't write - the Installer Log had a bunch of comments that made it look like it was trying to boot but something was in the way.
After googling this issue, I found a suggestion to remove the SSD again and hook it back up to my other Mac, reformat it to macOS again, and afterwards close Disk Utility and select the SSD in Finder. Chose "Get Info" and scrolled down to the bottom, and checked the box to ignore ownership (I think...maybe it was already checked and I unchecked it, I don't remember), see link where I got this idea:
I tried to clean install OS X El Capitan on my Mac, but it won’t boot from the USB drive. Here are the steps I took to create the bootable USB drive (I have a Mid-2014 MacBook Pro with Retina Displ...
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While there I also changed permissions so that everyone can read/write just to be sure.
Put SSD back in laptop, while booting held down option key to try to boot from USB drive. Took me to the grayish screen with four options - chose "reinstall MacOs" and selected the USB drive - this time it booted into High Sierra fine!
Once HS was running OK I downloaded the dosdude Catalina patcher as was suggested above (did it on my other Mac) - it downloaded OK and I followed its instructions and made a bootable USB thumb drive (different one). Repeated procedure of holding down option key on laptop, eventually wound up at the gray screen and chose "reinstall MacOS", selected drive with the Catalina installer - installed on second try (got an error on first try).
This was not a scientific study but it seemed like High Sierra wanted to install onto a drive with HFS+/MacOS extended. MAYBE it was because my thumb drive had been formatted to HFS/MacOS extended. Also I don't know why changing the ownership on the SSD made a difference but it seems like it did. It's also possible that with an APFS formatted SSD with the "ignore ownership" box checked would have installed Catalina OK too.
Thanks to all who replied. Will play around in Catalina for a while but also thinking about downloading the patch to upgrade to Monterey or something just for fun.