Ny Project Management FLOSS that has Resource leveling

You can get something along these lines via the Thunderbird Conversations addon. I haven't used it recently but when I did (a year or two ago) it was usable, though not quite as smooth as gmail itself. It is being actively developed, so it has likely improved since then. Reviews seem to be mostly positive, though it may not work with all versions of TBird.

You could pick one from the server list. Probably jabberd2 is closest to what you are looking for.