While I don't use a paper ledger, I do a similar bookkeeping with Microsoft Excel. 

It's flexible, cheap (no membership cost if you already have a Microsoft office account), no privacy concern, etc.

Microsoft, Apple, etc.   

All office software suites (Microsoft Word/Excel, Mac Pages/Numbers, Google Docs/Sheets) offer this functionality.

It sounds very custom, but I think all Excel OS Microsoft 10 and newer have a built in Amortization Schedule that you can start with, maybe make an original then copy that sheet and in the second sheet play around with it.Very Unique ask and would love to hear if that goes through and how you set up that type of payment!

It sounded to me that it was as bad as trying to pass for a Microsoft Certification test, which is why I ended up studying Linux distributions instead haha and just got a degree rather than pay for a test certification like my A+ computer repair certification.

I'm a big fan of paper and pen.I used to use Microsoft Project.  

You can use Access through Microsoft to store you data and use it. and you probably have it in your computer. 

As for now I have nothing but some paper, pens and Microsoft Word on my computer, ha.

A lot of years ago, I wrote a technical paper on this topic (long before there were blogs or MicroSoft Word).