I hear you.
I've made hundreds of changes to my site., but most are in the template areas. However, I have made a few changes to the core, but none have involved updating the database to my knowledge. Most are just cosmetic enhancements.
I tried keeping a record of the core changes, but they got out of hand. Before I replace a core file from Github, I compare it with the file on my site. So far I've been lucky and the changes are minimal.
I am concerned about an upgrade, especially to PHP 8, because it also involves "several" third-party modules which probably won't be 8 compatible.
With regards to the updater, is it really worth the effort you're putting in? GG's old clients may need help, but I somehow think there weren't that many. Because the forum is so quiet, it implies that a lot of people are seeking professional help, or they've gone fishing.
Professionals shouldn't need an updater and I'm wondering what problems I'll have using one. Could a more mechanical approach be taken?
For example, a package containing the cluster of files needed to upgrade the core, with changes highlighted (Github style for example) and a database dump necessary to update the database? The same could then be done for each of the modules based on their popularity. I use about 4 Cheetah modules and I feel that others may only need a few.
I thought the Boonex method was flawed and it nailed me a few times.
In a nutsell, it would be remarkable if my site could be updated to 8, but I'm happy to beta test if you need a guinea pig.