Helping move Templot2 to the latest supported Delphi platform would certainly be something I would like to help with, but it will have to wait for my retirement in a year or so. I fully understand your issues with data types and storage having had to back in the 70s write everything (databases, screen drivers and plotter drivers, etc.) for 2 & 3d seismic mapping software. This was used by most major oil companies around the world as well as the UK Dept. of Energy and BGS.
Today I would recommend moving to JSON for file based storage, settings, etc.
I have now installed the Delphi Community Edition and tried opening Templot2 in it. Needless to say there is a pageful of errors and warnings to work through. That will have to wait for now.
It's the full complex Delphi Studio interface -- I can't see TemplotMEC users preferring that to Lazarus. Also the licence is free now but has to be renewed annually -- there is no guarantee that it will always be free, Embarcadero are clearly using it as a loss-leader with nags about upgrading to the full versions.
I was also a bit surprised that I had to override the dpi-aware setting manually to get a crisp display.