According to GDPR, you cannot make access to your service require opt-in to tracking, unless your service very specifically cannot work in any way without track

According to GDPR, you cannot make access to your service require opt-in to tracking, unless your service very specifically cannot work in any way without tracking. And no, your business model not working because you cannot track users anymore is not the same as your service not working. Find a better business model. Facebook would still work just fine without the tracking, as proven by the fact that you can switch off targeted ads and everything still works fine.