Having given this a bit more thought, it should be clear that the strategy of entering Germany via Belgium or the Netherlands is doomed to failure, unless there is a complete relaxation of rules in the near future.

Firstly, at some point if you are going to get into Schengen, you'll get a Schengen stamp in your passport. To get in to Germany via these two countries (presumably with the car you rented), you'd have to hope that the border is open on both sides and you are not stopped, especially on the German side. As I've said above, that is not guaranteed. You'd then have to hope that you are not stopped by police during your entire time in Germany. With a foreign license plate, I'm not sure you can avoid that. And the hotel you stay in will probably ask for your passport. At some point you're going to have to leave Schengen. You won't be able to do it from Germany (the BP at the airport will see your entry stamp from when you were not permitted to travel to Germany, you'd then have to lie about when you entered Germany, possibly having to show evidence of this, which you won't be able to) or tell the truth and face the consequences. Which means you'll need to go back to where you came from, which in turn means crossing the border again.

I think you need to face the fact that this trip will not be possible until non-essential travel is once again permitted.