[toast0]

I would imagine it would be simplest to administer if you treat it as an installment sale.

The price and number of shares is fixed, the investor has a legal obligation to make future payments, the company has recourse if they don't.

Although, you have to expect any recourse to either be untimely or unsatisfying for the company. If the investor wants to stop making payments, they will, and going to court will take too long. So, your terms should be explicit about what happens when a payment is missed, and you should try to make that as wrapped up as possible.

Alternatively, maybe set it up as options expiring every 3 months. Cause that's more honestly what it is.