Can't help you with your seat tube size, and switching to a tubeless tyre set up aside, fitting a dropper seat post has been the best mod that I have ever made to any bike.
I even bought one to go straight onto the new bike, before it had even arrived.
Glad to read that you are also enjoying riding off road. I've never understood why people buy e-mtb's then don't use them off road.
If you want to learn more about them, or any other mtb related tip, this guy has a brilliant YT channel.