I think we need to define the problems. It sounds like the industry changed. Get business processes or business data requirements first. What do they want to k

I think we need to define the problems. It sounds like the industry changed. Get business processes or business data requirements first. What do they want to keep, change, get rid of, or add? Template out your business objects and data structures. On the topic of a single DB... I wouldn't. Unless your clients are sharing data, that's just a huge security risk to keep it all together with website accessible from anywhere. Then make your changes gradually. I assume you have no tests to look at for requirements, otherwise that would be helpful. It might be possible to migrate some pages or functionality by pulling in the new stuff using some sort of hybrid routing or microfrontend. Any way you look at this, these sorts of rewrites are painful.