> Hypothetically some simple restrictions for variable font sizes could keep the simplicity of TUI while also empowering the UX.

The only way variable font sizes work without being isomorphic to a GUI is if the sizes are a series of power-of-N multiples of some base size for some integer N. (Realistically, N=2 is the most likely, though N=3 might sometimes make sense.)

The reason for power of N is that it assures that any set of available sizes used still fit on a grid of the smallest size actually used.