Hi Michael,
If remote desktop like VnC or RDP is what you are after, then Google's Chrome Remote Desktop is one such cross-platform tool to use. I had tried it some time ago to GUI login, from an Ubuntu PC or Chromebook, into another Ubuntu PC remotely. Both sides just need to have Google Chrome browser as well as the Chrome Remote Desktop App installed.
I remember the remote Linux PC that you want to remote into just need to start up the Chrome Remote Desktop app from within the Chrome browser, and then just sit there ready for you to login from your end. There are security options that you can enable for the user to decide who to let in. Using Chrome Remote Desktop apparently have no problem connecting over NAT'ed network. Both ends just need to login to your own Google account will do.
H.T.H.
Wen