I plan to obtain a ham radio license in the near future for covering long distances (over the hill and down the valley).
Any recommendations on compact, reliable, and easy to install mobile ham radio units?
The possibilities are almost endless. Which radio can also dependent on how you want to use it as a ham, such as with DMR, DSTAR, Fusion, APRS, etc. (all modes and functions of ham radio) Your specific vehicle has bearing on "easy to install".
If you're looking to start with simple VHF/UHF (personally I recommend starting with a dual band, provides more options from the start), I like the Yaesu radios. I have an FTM-400 in my Jeep and an FTM-300 at home, but could also easily be put in the Jeep. Both have a head/control unit that can be separated for mounting in/on/in front of the dash while looking out for airbags, visibility, etc. And the main radio can be elsewhere, mine is under the passenger seat. I'm sure Icom and Kenwood have units with similar features as well, typically hams have their favorites.
This thread
Let's see some HAM setups in your rigs has a bunch of variety in vehicles, radios, and their setups.