Not a requirement, but a preference.

Onlyoffice looks like it might be good, I'll give it a try.

Can't stand libreoffice, it feels much like Office 2007 which was the worst version I ever had to use — fixed with 2013 and 2016, but libre hasn't caught up.

E: Found freeoffice which looks to have much closer parity to MS Office. I don't have a problem buying perpetual software licenses in these situations. I'd prefer FOSS, but for productivity software it has to be conducive to getting work done.

Collabora has also released a desktop version. I've been giving it a go and its UI is pretty nice, but its still fairly buggy at the moment. Keeping an eye on it for sure.