For commissions I think you should focus on drawing in a way you feel you can consistently recreate, mostly so people know they'll be getting something similar to your example images or previous commissions you've done. Headshot/bust/fullbody or whatever amount you're comfortable with is fine for format, imo. You don't have to draw in exactly the same way each time, but people should know roughly what the art they order would look like. You can offer art trades if you want to focus on drawing other people's characters in a consistent style, without having to do free requests.
It might take a bit of fiddling for you to come up with formats/prices that work for you, but each time you're going to add an option, just think "would I be ok with drawing this over and over for different characters, for this amount of money?" If you don't feel like you'd enjoy doing for example coloured fullbodies, simply don't offer them, or up the price to make it worth it to you if you do get those requests.
I don't have much advice for ko-fi or patreon as I don't use any of those, unfortunately.