I suggest you experiment with temporary attachments until you get things the way you want.

I like to use 1/2 inch webbing with 5/8 inch quick release buckles. Webbing is plenty strong and the undersized webbing adjusts easier than the webbing that the buckles were designed for.

Buckles and webbings can be tied to bags or frames quite easily using light cord. Buckles can be kept attached to webbing by tying a knot in the end of the webbing.

Adjustable quick release buckles will really help with the experimenting. It also allows you to take the front bag off and on easily. You may not want to wear it all the time. When I'm going up steep hills I carry the front bag high so it doesn't bump my knees. When going down hill I carry it low for more convenient access.