I kind of have this setup, except the other way around (a desktop Mac mini, and a MBP15 as a travel/spare machine).
The solution for me, is to have a TB3 M2 enclosure with a 1TB SSD in it, and use that exclusively for work.
I use iCloud for keychain, mail, etc; I have my dot files (including SSH keys) synced via iCloud Drive, so that stuff is pretty much ready to use once the second machine is online.
If the mini goes tits up, I'd likely have to install some software updates on the MBP but otherwise it's a pretty seamless transfer.