I would do, more or less, as I'm doing with my reading history:
- User/Loan Date/... Fields you need for current loan
- Action chains to move user and important dates to another field (adding as note or some columns you can search) after the loan is returned. For example, something like: user1: 2025/03/24 - 2025/03/30;user2: 2025/03/20 - 2025/04/02
If you set user as a tag-like column, you can query at any moment, which are the books a use has loaned (I guess you've already thought about this, but it's an easy way to control number of loans).