Should I pay myself back with a guarantee or an overhead fee when I make a loan to my business?

You must be very careful about this type of loan, as doing this wrong can have profound tax effects. This is called imputed interest penalty. So the IRS can determine a loan was paid back incorrectly + ascribe some random interest rate, then require you pay back taxes + penalties on whatever rate they make up.

Best way to deal with loans - Avoid any fancy nonsense.

If you make a loan to your company, really make a loan.

This means you have a note with market rate interest, so if you write a loan with 1% interest, this likely won't pass an audit. Make the loan match market rates you'd get at a bank for a similar loan.

Also make sure your loan is less than usury rates in your state. For me, in Texas US, this is 18%, so if I write a loan to someone for more than 18%/year, I can end up in court via a suit brought by the state's attorney general.

Summary: When making a long, make a real loan, with a correctly written promissory note.

Exception: If you're using Transfer Pricing + locating your IP (intellectual property) in a different jurisdiction than where you pay tax, then you have many additional tax optimization strategies available.

Real Example: Last time I checked, Google's IP is held by a Bermuda IDC + licensed to other countries for use. This means licensing agreements can be modified on the fly, to move any profits out of any country to Bermuda, where corporate tax rate is 0%... so profits can be expatriated to Bermuda, so no tax is paid.