You've removed everything that should be removed.
The wheel hub is centered (on the tractor's hub) by a raised section on the hub that fits into a matching counterbore on the wheel. It's a hand press fit...and should be easily removed by hand.
In your situation, the wheel is probably "rust welded" to the hub.
At this point, I suggest spraying a penetrating oil as best you can between the hub and wheel. Then (after a few hours) start tapping the rim again (rotate the wheel as you tap). You might need more then a 5 lb sledge hammer. Use some muscle and keep rotating the wheel as you hit it. (BTW, it might be advisable to use a short length of 2x2 or 2x4 against the wheel and strike that rather then the wheel.)
A puller might work if you pull the center cap off. I've never had to do that so I don't know what's behind that cap (probably a seal and a bearing).
When you get the wheel off, make sure you apply an anti-seize on the hub before putting the wheel back on.