Thanks for sharing this. What if we were to continue to calculate interest and fees using our current flat method for the time being, but display the equivalent APR to lenders and borrowers? As a prospective lender, would this have addressed your concerns about transparency?
I work in the US mortgage industry where advertising of "APR" rates is mandated so consumers can easily compare different offers. Mortgages are still permitted to have all kinds of odd financial arrangements: fees and fee-like structures (points), negative fees (cash-back refinancing, negative points), different payment schedules (interest accrued in advance or in arrears, monthly or biweekly payment schedules), and much stranger things (balloon mortgages, even reverse mortgages), and also many things vary from loan to loan (for instance: some costs vary according to which county you live in).
All of that complexity gets baked into the APR. Fees are amortized over the life of the loan (or sometimes a shorter period). Various kinds of rates get boiled down into an annualized APR. This is far from perfect: no single number can make all that complexity truly comparable, but it roughly does it's job. Each individual loan has an APR calculated once its rates and fees have been set, and marketing messages must quote APR using a sort of "typical loan" for that lender. I think you could develop a definition for "APR" that handled the flat payment method but made it comparable for someone like me.