There are several paths you can take, but your problem points to a faulty solenoid in the starter. It's not getting enough power to fully engage and is staying in the high current draw mode, trying to pull in fully. You might be able to fix your problem with a new starter. There are plenty of sources for a starter for your tractor, all under $100.
Here's one example on Amazon. What's happening is that you've probably increased the size of fuse #2 until it is 15 amps or more. With repeated attempts, your ignition switches contacts get hotter and hotter. Finally, the switch burns out because it has a plastic body.
You can change relays, but a better solution might be to see if there are two relays of the same kind in your tractor and swap them. If the problem remains, there's a good chance the relay is not your real problem.
With your starter, it has two terminals that are very easy to get to. Have you ever heard of a remote starter switch?
Here is one that I bought from Amazon. With your starter hooked up normally, hook one alligator clip to one lug and the other one to the other lug. Press the button and your starter should engage and start working. What the button does is what's commonly called a hotwire start. It just connects the solenoid of the starter to +12 VDC directly through the switch instead of going through the ignition and any relays. If you try this and your starter does not operate, then you may have bad leads coming from the battery or a bad battery ground connection. Of course, it could also be a faulty starter, but you'd probably hear something in the starter, at least a click.
Since you have a history of starting problems, I think you need to buy a remote starter button/switch and give that a try. You've already replaced too many parts to keep doing what you are doing. Just remember that the remote starter bypasses all safety circuits. If you have the key On, your engine will start when you use this switch. If the key is OFF, the engine will still turn over, but the tractor won't start.
Would the relay conversion work on your tractor? Maybe. . .you can't be sure until you try it. Do you need a new starter? Maybe. . . you will know more if you use the remote switch. All you really know is that your starting circuits are not working and that you are spending a lot of money on parts repeatedly. You have to decide.
