Take a look at Delta fans. They are inverter drive and have a very high power density. One would work nicely for the bathroom exhaust.
Here's one for under $15. You'll also need a 12V power supply capable of supplying at least 4A, of which an old PC PSU works nicely. And a simple 555 circuit if you want to run it at less than full power at some times.
https://smile.amazon.com/Original-De.../dp/B00L5XEKOS
For the two big windows, most likely the most efficient configuration would be a box fan blowing in on one window and just leave the other window open.