Ah ha, so I think I found it. MobX. I've used it with React. In fact, it's what is being used by my current website. MobX has the benefit of not being opinionated and since I already have experience with it, I already have a head start on the learning curve.
I just need to figure out the integration piece with Lit.
Ok, so I think I'm set on the tech stack for my new web site.
Ironically the only major change after all that talk about refactoring was switching from React to Lit. 😅