I like the AAA WCAG recommendation. I'd also recommend from my casual experience listening to lots of old people...

- a large font size by default, and maybe a font size slider on the homepage. Test everything at 200-300% scale as WCAG recommends

- don't change the UI! Or change as little as possible, at least for existing users. Which kinda upturns the whole always-updating nature of web SaaS but I think it can be done

- hire a good designer who can streamline your UX and screens and keep only the bare minimum features

- maybe offer human support? Like a phone number? Probably unreasonable for you tho

Wish I had ideas for simpler login and auth.

Have you found any successful design strategies in your 10 years? Any insights from user testing?