As regards deleting topics, it would have to be all or nothing. Making purely arbitrary, obviously-biased decisions as to which forums to delete would be against the 1st amendment. Of course, if all the forums got deleted and then renewed, there would be a genuine refreshing/new blood, without any obvious inbuilt biases being kept.
I'm not clear why you're worried about the 1st Amendment. This is an international forum that isn't bound by US Constitutional Law.
Regarding deleting all forums and starting fresh, as I think about it more I have to admit that idea is growing on me. Especially if we change the topic areas. Maybe have sections for specific types of foods (fruit, vegetables, meat, etc.), sections on various health problems (heart disease, cancer, autoimmunity, etc.), a section for people's personal stories and/or testimonials, and another for food preparation tips. Get rid of the 'off topic' and 'hot topics' sections to cut down on posts that might reflect poorly on folks who post here. We could make some of the sections public and others private, making it clear that to see everything you need to sign up with a valid email address. Then we could do birthday emails, reach out when folks haven't posted in a while, and other such things.
On payment, there are benefits and drawbacks. If this were a thriving forum that members clearly valued, charging a nominal fee could raise enough revenue to offset the costs of the hosting service, and perhaps provide moderators with a stipend. This forum is far from thriving. Indeed it's just about dead. If you instituted a fee now I suspect you'd get a handful of people who paid the fee, and post frequency would decline accordingly.
I'm part of a Meetup group that, two years ago, had over 500 members. The group's owner instituted an annual fee of $5 so he didn't have to pay the cost of the group (about $150 per year) out of pocket. Membership quickly fell to about 25 people, and has hovered there ever since.