Ah, yes, if there are other transmitters nearby or in your target area then that complicates matters considerably, especially if you can't dictate what channels those use.

Another option then might be to have a spool of fiber and to use that instead. Fiber will happily go longer distances without much chance of interference or repeater issues. It's a more expensive option than copper but if you need to go over the easy-to-do-with-copper distance that might be a viable solution. Best of luck with this, it's funny how doing the last kilometer of a link can be harder than getting packets from here the other side of the Atlantic. Very easy to take infrastructure for granted.