I've cut them like that til they were standing up again where I could push them over. Some stuff in the 15" range. It takes a while to do that though and lots of reading pressure on the wood. Is there anything else in the area you need to take down that's big enough to smash that to the ground?
I deal with ones like that quite a bit. My preferred technique is to use a felling-type cut with the notch in the compression side, which is usually the top of the log. I find they 'kneel down' more in a more controlled way, giving you plenty of time to step back and see what it's going to do once it starts moving. However, it only works if the base of the tree is free to move, and it sounds like yours isn't. In that case, the bucking strategy described by Dennis works better. But that technique can release the tree quite violently (see Butcher's vid), so I like to be really sure about what it's going to do and where it's going to land. I'd probably go with the bucking method for the first cut, and if the rest doesn't get too stuck in the ground, go back to the notching technique. Also, to be clear: you don't want to make these cuts as firewood sized rounds - both techniques depend on leverage and gravity to get everything moving. I usually measure out some multiple of my split length to figure out where I'm going to make the cut - usually at 64" or 80".
Forget the lawn tractor. As you indicated, it will not budge things. Use a come along attached to a nearby tree as your pulling tool and make those 4 or 5 foot up cuts to break the trunk loose from the soil.
What's worked great for me is what I'd call lateral felling cuts. In the worst case, where the butt's fixed in place, I make sets of face cuts & backcuts, one as low as possible, another 4-5' away. I make the cuts with the bar largely vertical, and leave decent hinge wood. The "upper" set of cuts has its face looking away from the direction the winch will pull it. The set of cuts at the butt face the opposite way. Pull with cable winch/chains/straps above the second cuts and the butt can move pretty easily, unless the stem is under lots of compression. Felling wedges can prevent pinching there What's worked well with big angled stems with their butts pressed into the ground- I just cut through from above, setting a couple of felling wedges in the kerf ASAP. The simple cuts are made straight through at 90 deg., nothing like Murph does, with the saw no higher than my chest. On finishing the cut, the stem pops free. As necessary, the "lateral felling" approach, pulling a section out from the butt can work too. Triggering it from a distance with a winch can keep the pulse rate down.
