Best way to track visitors to my handfull of sites?

I have a couple of sites with similar themes offering e-books and a SAAS service.

In order to pay affiliate commissions, I want to track users across the different sites. 3rd Party Cookies are obviously no longer a good solution for this.

What are other options?
The next best thing I can think of is populate an ID in the links across the different sites. This would obviously be a lot to keep track of, but I’m not seeing a more reliable option.

Any other suggestions?

Also in #Full-stackDeveloper _↴