We have Mac computers from 2008 (OS 10.11) thru 2019 (OS 10.15) here with others in between (10.12, 10.14 ...) and have dealt with the iPhoto => Photos version compatibility challenges for some time. The latest change to Photos I feel was not value added, because the loss of the Masters internal folder which had all photos with their original names (from the camera) and ordered in folders by date was helpful, in the previous version. Now the naming and ordering is not discernible, it is hidden in a data base file. My daughter, a professional photographer, used to use Photos as a storage/repository for the original raw photos off the camera cards, she would then easily scan through them and do her serious work with them in Lightroom. Well, no more, now she has abandoned Photos as the raw storage repository because the tools she was using to parse the image folders to feed into Lightroom can't parse the new unfathomable folder structure. Instead she just uses Image Capture with the camera cards and puts the raw images into finder folders that she has logically named by date just like iPhoto and Photos used to.
With the previous internal folder structure, I think there was a chance for both forwards and backwards compatibility with Photos, but now of course older OS's cannot read the new Photos libraries. This is disappointing and in some cases inconvenient, but not surprising to me. After all, older versions of MS-Office, Adobe programs and many other tools eventually lose the ability to read the newest program files.
My only suggestions for working with Photos in High Sierra and with Catalina, assuming you cannot make use of the iCloud sync process due to bandwidth:
(1) Make the Catalina Photos library your working library and periodically "export" all the photos out, then reimport them into a Photos Library for High Sierra. Time consuming and inconvenient, at best. And you lose some customizations.
(2) Make the High Sierra Photos library your working library and periodically duplicate it so that Catalina Photos can work with that duplicate library. But this is also time consuming and inconvenient because then Catalina Photos has to go thru its "cataloguing, sorting ..." process every time you do this.
(3) Neither of (1) or (2) offers a convenient way to synchronize changes made to these now two separate libraries.
(4) Look for a different tool from Photos that will work across the 10.12 to 10.15 operating systems using the same data base. For example, I believe one of the versions of Lightroom Classic runs on both these platforms with the same data base. Lightroom is not free like Photos is, but it is not very expensive either and the license allows for it to be on two computers.
I don't know why Apple did this to the latest Photos. It's Apple's prerogative, but I think most consumers simply use Photos to store their photos and get rid of red eye, and maybe do some simple editing, cropping, lighting adjustment, etc. I don't know why the data base had to change so dramatically -- that's what has led to the lack of compatibility between versions.
