If the coax is buried, you should not need any further decoupling. Any
RF current on the outside of the coax should end up amost entirely on
the ground.

I once had a 4BTV roof-mounted and didn't have many radials on it. I
didn't need decoupling for most bands, but 10 meters was downright
awful with high RF at the transmitter. I solved it by winding a few
turns of the coax to get a choke that was self-resonant at 10
meters...I seem to recall that I might have actually put a little extra
capacitance across the coil to tune it, but it was a long time ago.

If you put a neon light next to your coax near the transmitter and it
lights up when the transmitter is sending power to the antenna, it's an
indication you have lots of RF on the outside of your coax, and
decoupling is in order. You can do the same thing with greater
sensitivity using a small coil feeding a field strength meter...high
fields near the coax indicate current on the outside of the coax.
Cheers,
Tom