I should mention in case you need a rough number that my 6m/20 ft yurt with 6 ft walls has ~680 sq ft of fabric area, not including the toono flap. This
website makes it easy to get the surface area of the roof (lateral surface area). Walls are a simple 3.14*diameter*height, minus the door area.
When I was wondering about waterproofing, I checked out a company called "Otter Wax". Quirky portland shop, mildly hipster but intriguing natural wax waterproofing. I don't know how it would turn out on felt, but they have melt cans that are brush applied. I tried it on a canvas scrap and didn't notice much improvement from the inherent water repellency of the canvas so I neglected to apply anything. Give the canvas a few years in the sun & it might be a different story...
The other waterproofing I've dealt with a little is Nikwax, a company that specializes in outdoor gear waterproofing. They have quite a number of different formulations--one for cotton, one for leather, one for synthetic fleece, for instance. I don't think they sell in gallon volumes, unfortunately.
You might want to get a few small volumes from different companies, then test each for how well it applies and how well it repels water on your felts--might take ~$50.
My canvas is keeping the water out alright, but it might need a hand with the more frequent winter rains. My door is the main problem right now--in a good rain water seeps though some of the paneling. It was stained but not sealed against water, dried out a bit and now the silicone caulking needs to be redone.