BTW you aren't supposed to be able to put your feet flat on the ground when on a dirt bike. Tall = good:)
I think you'll find that the firmer the better for skinny seats, otherwise the wedgie thing only gets worse. I did find that my YZ was much more comfortable after I made the seat flat (rather than rounded) with a belt sander. I started with an extra tall CEET seat for it then flattened the middle (but didn't reduce the height any significant amount, just took off the crown). Made it much more comfortable
Oh, my old DR's have sight glasses and little plastic tool holders, they are nifty aren't they
You should set the rear sag, its sorta like tying your shoes or something - just something that everyone needs to do. The chassis and suspension won't work properly without the correct amount of sag. 95mm would be about right for a starting point.
have fun with it, looks to be a very fine machine