The TripleDown and TexasAvenger use a FET for Turbo and 1-6 7135’s for high mode, then PWM’d for lower modes. It is a bit of a compromise mainly limited by space on the board and the amount of pins (8) your common Attiny has to offer. Of those 8, normally one is used for power, one for ground, one for mode switching, one for programming via clip, one for voltage detection, and the other 1-3 for controlling the output regulators.
I think MikeC has done essentially what you are thinking of Lexel, but it didn’t make much buzz because it is a little complicated.
See here: