Some options not mentioned:

Since this is an RDP user interface, maybe rofi is more appropriate than dialog.

Since json is a pain point, maybe compile json-5 or relaxed-json master into a separate config for each server, then rely on aliases.

Passwords in the clear aren’t great, but if you’re scripting then you have the ability to only write a sensitive config (say, from your password manager) and delete it after forking rdp.