I did not suggest you should. I told you Excel has functions for this.

Many (probably most) do. But when you make extra payments, all additional amounts (usually) go to principle. And that changes the interest calculations for future payments.

Usually true, even if you do not make additional payments, because you are paying interest on a declining principle balance. (Some loans are structured differently, but not most.)

You will never be able to enter a single line payment, because each payment will be split between principle (the loan liability account) and interest (the expense account). So you always need two lines, no matter how you determine the numbers.

Using standard financial formulas based on principle, interest rate, and number of payments, easily found online or available on some handheld calculators or in a spreadsheet program. The question is how the loan documents define the calculations. Is interest calculated daily? Monthly?

Unfortunately, yes, unless the lender sends monthly statements with breakdowns and all payments as of a certain date are considered to have been applied on another specific date.

Not at all. I readily admit accounting for loans with constant payments is a real pain. While the basic concepts are straightforward, matching your calculations to those of the lender is seldom simple, because so many things in the basic formulae can be interpreted so many ways. The one thing you know for sure is that the lender will interpret everything to their advantage, not yours. Sometimes, you have to make the payment with your two line items, then go online to the lender’s web site to look up your actual balance and back out the numbers after the fact to edit the transaction. (You’ll only be editing the apportionment between principle and interest, not the total.)

Sure. Your method requires two entries: the payment and a journal entry that can only be entered after you somehow determine the interest amount. It’s easier to record one payment with two lines. The process of determining the exact interest versus principle will be the same. It will depend on the terms of the loan and the resources available from the lender. Your method implied that interest might be posted on a different day from the principle, but that does not happen. They are posted simultaneously by the lender. So why not combine the transaction?