That would not be excessive to me! You might want to research the cost of an engine rebuild and I doubt that you would think the oil consumption is all that bad either, especially if the tractor starts and runs good as-is.
I am not all that convinced on synthetic engine oils, especially in engines that were not designed to use synthetics. I think if your tractor was mine, I would switch over to a less expensive, diesel rated conventional oil and just plan on adding a quart here and there as needed. As for oil changes, what you have been doing is probably all right unless your tractor is often used for very short time periods, so it never gets a chance to come up to operating temperature to boil off the moisture in the oil. Good luck!