I am not sure you can use March as the monthly sales averages RIGHT NOW, but possibly after a few more months, it will start to come back to those numbers. The reason you can’t really do this is because you have to spend money and units to get to page 1. How will you be able to do that if you drastically underestimate the current market volume?
I will say this, in times when you see these massive spikes due to COVID-19, you will have to keep watching this on a monthly basis to see how it goes, but ideally what this means is you have found a product that is riding a successful wave at the moment, and if you can AFFORD to jump in, that should leave you in a good spot to get some of the increased volume.
However, if this means it is out of your budget for now, then you probably have to miss out on that opportunity and find one that is within your budget.
The rest of you analysis is quite good, in my opinion. I think it has good potential, but the increase in volume now may mean that the promotional units exceeds where you can afford to compete.
Try adding those numbers up and see what the First Order cost comes out to be.