Adjustable Phase Shift
I doubt you could use a trimmer cap in that frequency range. At those frequencies everything becomes extremely sensitive to parasitics and probably you would need specialized microwave circuitry.

Anyway you could do with a module like this. Excerpt from the app note:



I just googled for digitally controlled microwave phase shifters. If the digital control through USB seems overkill, maybe you could find some pre-built module with a more "low-level" interface (e.g. SPI, I2C).