What about from a UX perspective? Giving the user control over how the site works and looks is hugely beneficial.

Only if those controls work. Which they make no guarantee they do, and which are not necessarily grounded in real research.

Microsoft recently discovered and shared their finding: “Nothing about us, without us.”

Microsoft did not discover that. The phrase dates back to the 1990s when, in addition to other prior uses, it became a disability community rallying cry.

…but is there ANY benefit to AccessiBe (or any of its ilk) if we go into it with the mindset of enhancing the UX rather than full WCAG/ADA compliance?

Given the inconsistent output, extra load for end-users’ browsers, lack of broad research supporting the changes it makes, and the fact that just embedding the control increases your risk of WCAG violations (by adding more), I would say no.

Instead, do some research with your current users and find out what, if any, of those features might benefit them, and either build them into your site or adjust your design to incorporate them.