I think the rule of thumb is it should be the same or higher than inflation. Whatever the government says inflation is, triple that number for the ACTUAL number. Set interest rate to that. Perhaps slowly over the course of a few years. Back when interest rates were double digits, stuff was also way cheaper than it is now. In the end even accounting for interest paid on a loan, houses were way cheaper, and so were cars and other big ticket items.
We need to stop normalizing credit being so cheap and easy to get, that just makes it so it's normal to use credit for everything and then companies know they can charge more because lot of people are dumb and only look at how much it will cost per month. Cars for example. It's really insane how expensive they are getting. Especially EVs, and that's suppose to (and should be) the way of the future, but at those prices they're just not accessible to most people. If loans were like 20% interest then they would have no choice but to drop prices on cars because most people are not going to be approved for a loan on such an expensive item at that rate. When car companies offer 0% financing, they're not doing it from the goodness of their heart. It's just built into the price.