> A fairly simple workaround would then be to charge a monthly fee instead, and you would then end up with something that in practice is more or less the same.

No. Islam bans all these workarounds. What you describe is still a money for money exchange. Interestingly enough, Islam does not just ban interest, it has a concept of Riba, which includes interest/usury, but encompasses other transactions as well. What you describe falls squarely under Riba.