There are a couple of thought processes with how your deck is supported. The two are, first is the supports close enough together to keep your joist from sagging and second, are your post stationary enough and how are they handling frost heave.
You should be able to find some tables from deck building that states the proper floor joist for the spans that you have. Another way would be pull a string along side of the bottom of your furthest span without touching it and putting weight on it and measuring the deflection of it. You would need to determine what a reasonable maximum weight it would likely encounter and put that on there. With your bad knee(bummer), this doesn't sound like a great option for ya.
Second, your post where just set on the ground, bummer, kind of a poor practice and it is pretty easy to make holes for your posts when you're building the darn thing. Just setting them on the ground might be OK if ya really level ground that drains nicely. It's best to have post below frost line to stop any heaving. Having a yurt like yours is pretty cool cause a little frost heave here and there won't likely hurt a darn thing except make the door stick if it's heaving in that area. There are many opinions on how to back fill posts, from using concrete, gravel, sand, gravel and concrete, you name it, it's out there. If it were me, I would have put post pretty deep in ground and back filled with gravel and maybe brace some posts with some posts angled at a 45, if ya know what I mean. Every site is different, drainage, soil type, etc.
More supports might not be needed, just remove some that ya have and place posts deeper. Fun with that is getting your post in there and tipping them in place without them being too short. I would have to scratch my head little and find a creative way to make that work, like maybe jacking it up a smidge higher than needed so when you come down it won't be too low.
When you shim it, give yourself the option of boards of many different thickness. If you have a bud with a table saw or planer, you could make 5/8th, 3/8th's, etc. I guess my anal tendencies are coming out of me doing too much finish carpentry in the past. Not sure how much compression you will get, make sure your shims are good and dry and you could choose a denser wood and that wouldn't compress as much.
Don't know what type of soil ya got to dig but I use a twist type post digger here in my nice soil and I ya wanted, you could put different length (shorter) handle on it. Also, I've used a bottle jack for that in the past and it worked nicely, don't know what kind of jack that you have available.
Good luck, I think you'll get it figured out.
Jeff
