Long story short, I gave the upgrading another go. After a lot of unnecessary struggling, I was able to successfully bring my Enthusiast installation up to speed and compatible with PHP 7.3 thanks to this updated version of Enthusiast.
That, however, came at a price: I lost the customized members list someone had put together for me on the original Enthusiast. If I have free time left to look over and compare the original files, I can probably integrate the PHPFanbase-like members tables again.
For the time being, upgraded vanilla Enthusiast will do. I'm just happy enough that my site can run on PHP 7.3 now. I will continue to manually update the fanlistings since the forms have made things easier to check.