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

Really just depends on whether my wife finds something her family wants or I do. Throughout the year if you see something your SO’s family would like, buy it and let your SO know

As someone else called out, check out Bazarr