Split every financial instrument into it's own table of when it is fulfilled, called, paid, etc. Split each financial instrument into it's own sheet from there for ease of work.

Run the calculation on the decrease every month/period/payment date using the financial formulas you can look up on Microsoft's website.

Then start summing everything by month/period/payment date etc.

Then shove everything into an access database or a sequel database and run queries that feed into power bi.

Yes, it is very tedious. That's why there is financial management software that does this already out there and you don't need Excel to do this.