80% of the way through a massive refactor where I am neck deep in, about 15 full days now, assisted by opus 4.6.
I’ve thought about doing this architectural change for the last 18 months but could not justify its cost, being alone. My experience is about the same as yours except that I am creating a new prod, doing a lot of manual intervention in the plans and QA, and will migrate each tenant one by one to make it less dramatic. My tenants are fully isolated anyway so I do not have a real choice there.
For the record, I never planned that this specific app would become a real company, so I am refactoring it from single-tenant (« server closet » model), local storage, singleton processes, to a multi-tenant architecture, object storage, and horizontally scalable. Never planned for it to be sold to more than 1 client at the start 
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Creating a lot of automated « architecture checks » helped a lot. Making the changes as mechanical and TDD-driven as possible helped too. Without this tooling my other options were not doing this migration at all, or seeking funding, despite having planned it carefully for « when I will have help », my plans were only a huge github issue for my own attention, and an (un?)healthy dose of hope.
I also considered throwing everything away and restarting from scratch with a scalable model from the get-go but it wasn’t an option that I was keen about, for obvious reasons. I don’t like wasting an useful app.
