Its not entirely related to monzo, but I recently added my mortgage to my finance spreadsheet dashboard and replicated the mortgage overpayment calculator in google sheets;
It helped me come up with an overpayment game plan to cut 15 years of the mortgage term and save £30k. I can now see exactly how much interest my mortgage is gaining every day (~£10!!!), how much of my mortgage payment actually reduces my mortgage balance, and I can see the exact impact of overpaying today versus any other day.
It was a massive pain to set up and has slowed down my spreadsheets significantly as it has to calculate compound interest everyday for decades. But the added visibility and understanding of my mortgage is invaluable.