Counterpoint: I find it really controlling to take rent only to save it up and give it back later. By all means make saving up a condition of living rent free, but your "child" is an adult and should be able to manage their money, and should be left to do so.

If they can't manage their money, well, you need to teach them how rather than control their spending so they don't have to manage their own money, otherwise they will founder when they move out and you will be back to square one.

I think it would be reasonable to split bills and family food bill with an adult child, and if they have any particular items that are not shared with the house, they pay for them. E.g. their favourite alcohol if they don't share, their personal care products, clothes etc.

Whether you charge full rent, or partial rent, or no rent, is entirely up to you. I make no value judgements about that decision because it's so situational.