From an end user POV, you still probably need MDM, Perhaps there are free ones you would like?
O365 could be replaced with your own self hosted NextCloud setup. You still need email, and you should be buying it from someone. If you don't need things like ActiveSync/shared calendars, then you can get RS mail for $1/u/m, but really, most want those things, so you're at least at $4/u/m at which point personally I'd still go O365 hosted exchange only (but I wouldn't because I want my storage integrated into my email client. Which I suppose could be done with NextCloud (NextCloud has a built in web front end for any backend email service (for desktop use) - that combined with a mobile sync app I suppose could make that work similar to O365 with Sharepoint).