I think using the same extension for all mangas is ok if you get some backup extensions to migrate the manga you’re currently reading at will. The ones you listed in another comment are great.
I’m usually reading one manga at a time + the ongoing ones when a new chapter drops (via notifications, which is my initial need of Tachiyomi). From my personal experience (which is technically riskier than your suggestion, but more comfortable) with mangalife, in the 3-4 years I’ve been using it, having to migrate due to outage was fortunately not a problem.