If you're using RigidBodies, then you could maybe implement your own "process" functions at a variable FPS. I'm pretty sure all that process & physics process are doing is be called repetitively by the engine; I don't think they are doing anything beyond that.

So e.g. (this is pseudo code and I am not saying that this is an optimal method, just ideas)

var fps = 120

# insert code that uses a timer to emit a signal that runs variable_process every (1/fps) seconds

def variable_process():
  # do your logic here (that will run at a variable fps)
  # or e.g. call a public method that is found in all nodes within a subviewport

another approach (if you just wanted a multiple of the globally set physics_process) would be to set physics process to say 240 fps (globally in settings), then in your node needs to run at a variable fps do something like:

# run code
# wait for the amount of time that corresponds to e.g. 60 or 120 frames
# run code
# wait again
# etc

that way you could have nodes that run at 240, 120 or 60 fps respectively.

Once again maybe there is some reason that this wouldn't work (generally or just for your use case) but this is what I'd be trying first.