Nightmare upgrading to 1.5.8

[inventoryops]

I did finally get my site to work, but I thought I'd post to get any tips/suggestions for next time because this was soooo aggravating. I spent the entire thanksgiving weekend on this and several days the following week.
My site https://inventorybooks.com/ was on 1.5.7 and seemed to be working fine when I decided I wanted to change my PayPal account from Payments Pro to
Standard. When I went to the Payment Modules in setup, it only showed a couple of modules (I think it was just showing the 2 authorize.net modules). My site has been on Zen Cart for 20 years, and even though I couldn't access the payment modules, they were working.
I decided rather than spending time figuring out what was wrong, I would just upgrade to 1.5.8 and see if the payment modules showed up there.
I'm not saying I'm an old pro, but I have been on zen cart for 20 yrs and have done many upgrades during that time without any problems. I also have sites using Joomla and Mediawiki, so I've done this stuff before. My site was pretty basic, just using modules that came with zen cart. the only mods I had was to text and logos for emails and the site.
My normal upgrade flow was to download all files from my site, download the upgade file. Copy the upgrade files over the existing files on my PC, make necessary changes, then upload back to my site and run zc_install. I did that, and the site wouldn' load (it opened the page pointing to errors in configure.php files). The configure.php files were fine as far as I could tell, but it kept giving me those errors.
I then started trying every Idea I could think of to get it to work, including installing a clean version in another directory on the site, then copying that to the main site, and even installing on another site I had (on the same hosting account Siteground). I even switched from uploading a zip file to the server and extracting it there (how I have done it in the past), to manually uploading groups of files . There was always something that didn't work, everything from the template not loading, to the all producst page not working, to papal not working. the clean install I did on the other site was the closest to working, so I moved that to my main site, and, of course, more stuff didn't work. I was ready to throw in the towel, so I restored my 1.5.7 site from a backup and was prepared to just give up. But giving up was not really part of my dna, so I tried one more time to do a clean install in another directory on my main site, then started making the changes to get it to be my main site.
That worked... mostly. The payment modules were back and I was able to downgrade to paypal standard. But in some browsers it looked like the template didn't load if i cleared the cache. Also, I used to have quantity discounts that worked, and they no longer did. My zen cart site did a fair amount of transactions 20 years ago, but it doesn't any more, so this was good enough for me.
So even though I got it working (for the most part) I don't know what was wrong before or what went wrong with the upgrades. So I'm just looking for any input on options to look at in the future.