Hey Nathan,
Thanks for the quick reply!
After working with the hosting provider's technical support, we found out PHP extension nd_mysqli was enabled in PHP v7.4, but not in PHP v8.0 (by default). This appeared to be what kept me stuck on step #1 (of your advice).
So, after enabling that extension in 8.0, I was able view the ACP again. Next, I was able to upgrade to Community v4.7.6 via the ACP upgrade screens. Then, I upgraded to PHP 8.1, but was not allowed to enable nd_mysqli this time because a warning message stated "nd_mysqli skipped as conflicting."
However, the site and ACP appear to be working as expected, so I believe I'm good to go. Thanks all!