The way I have forward and back thrust configured is I use them in the Landing/Docking override section of the Controls config, as I don't really see a use for them otherwise.
I have buttons 7 & 8 (the top and bottom rightmost buttons on the left side of the stick) set to forward and reverse. As the name implies, those buttons have other functions when the gear are up, though.
Since with this setting you have to have your landing gear down it might not be as useful to you, but to me this makes the most sense, as I'm only using forward and reverse in situations where I need precise control like scooping or mining or docking, and having the gear down, which also lowers my max speed and makes boost impossible helps with situations with those a lot.
One other thing you might consider are shift states using the left hand buttons. This is what I do. In the Control Options you can press more than one button at the same time and get multiple functions out of a single button.
In my case I use the 4 way hat for 24 functions. Pressing (and holding down) 9 and using the hat sets my power allocation, for example, while button 6 and the hat controls the scoop, gear, and scanner resolution, while button 7 and the hat controls boost, heatsinks, silent running, and FSD/SC engage. The pro of this is you are using the same hand you use for the throttle to push buttons so you're getting more out of that hand. The con of this is you have to use two hands on the stick, if you're controlling throttle some other way than the stick's throttle or using the keyboard with any frequency this won't work as well for you.
But in general that kind of setup (any multi-button combo, not just the hat) might work for you with reverse and forward if you see the need for it.