FreeOffice is closed-source, but free, and available on Linux, macOS and Windows. It works pretty well and I think it's better than what LibreOffice currently offers.

[1] https://www.freeoffice.com/en/

> closed-source, but free,

That's the scariest combination possible IMO. Regardless of the trustworthiness of the developer, it stacks incentives in a bad way. Even if LibreOffice is worse, at least it'll be around longer than one single developer.

I agree completely, but it might suit the use case of someone trying to get some work done.

There is also WPS Office. https://www.wps.com/en-US/. It's actually very well done, and seems to be pretty compatible. It also runs on everything.

It's usable in isolation, but at least the last time I used it, it did not fit the parent post's criteria for "compatible-enough that they are realistic options"; it did break document layouts when exchanging work to/from people using MS Office.