Maybe you will find some fellow nerds interested in going deeper here: https://mess.foundation/

A creative hobby in which you could apply your programming skills perhaps?

There's no redundancy. Imagine you have 2 buckets, you reach into bucket A and grab some cash and put it in bucket B. $x dollars leaving bucket A is entry part 1, $x dollars entering bucket B is entry part 2. That's all it is. I honestly don't understand what single entry is by comparison - it sounds like losing an essential attribute of the transaction.

"Two sources of truth" is an anti pattern in computing and isn't redundancy. Use replication or backups for that.

You can't imagine simpler than what you described because it's single entry.

Double-entry is twice as complicated, makes sense only to accountants and not to computing people. Your example of 1 transaction would be doubly-kept as some nonsense like

  BucketA revenue:$x CR cash:$x CR
  BucketB revenue:$x DB cash:$x DB

no. single entry is when you track BucketA’s balance without regard to other buckets at all

Single would be recording the two accounts, one being the source account and the other the destination, and a quantity.

single entry is like a receipt at a store, is says what you owed and what you paid, but there’s no “other side” that is getting credited or debited the same amount

I agree that you are paying the $5000 rent whether or not you own the house - but with ownership you have also levered your downpayment through the mortgage no? Is it better to use your downpayment to get that leverage or to invest in the stock market with no leverage (assuming some fixed positive return on each asset)? How much higher do stock market returns have to be over real estate for stocks to come out ahead? Has someone modeled this out or written about it? I've been wondering about this for a while and considered trying to model it myself but I don't understand all the other factors so not confident I could do it correctly.