Bad news: I've run out of time, so I had to go with what I've built so far and push it from sandbox to production.
More bad news: I'm still working 50-60 hour weeks, so content will still be minimal at best.
Good news: It will get better. Now that I'm here (current CMS and PHP 8) I can take my time and get it all fixed properly and without a Sword of Damocles time-crunch looming over me.