Mint, (?)ubuntu, and Pop!_OS are what I suggest because 1) most software install guides target these distros. Anything that uses a package manager other than apt means extra googling and pain for those who just want an OS that works and could care less about the miniscule advantages of one over another 2) stable releases and driver support and 3) similar UI to whatever they’re coming over from. Someone else ITT mentioned KDE for Windows and Gnome(or Cosmic once it’s stable) for Mac folk and I think that tracks well

Damn, wish I had known about that before I went the takeout method

For databases, many like postgres have a ping / ready command you can use to ensure it’s up and not have the overhead of an actual query! Redis is the same way (I feel like pg and redis health checks covers a lot of the common stack patterns)

Encrypted Matrix or mattermost server with jitsi?

Depends how close they are. Most of my friends (that are into self-hosting and collecting Linux ISOs) are across the country / world. So mailing encrypted SSDs works best for large transfers and nextcloud for small transfers