I want to pay for it to increase the probability people can work on it in a sustainable way focussing on my interests at least to some extent. I also want a fully managed product working on the devices I use.

I am not sure since I didn't look for another product for a long time now. Is there something that provides the above while still being open source?

Because we value our time? Self hosting is a time sink.

Them being proprietary means they can invest in marketing and UX and QA.

That's how you get a company of 500 employees to buy your product that "just works" in all browsers, operating systems and mobile devices over a free one that needs every single user to fiddle with a custom solution.

Spending $5 per user and month is way way way cheaper than wasting 1h of an employee's time even once.

I used LastPass at work because work told me to. That was basically it.