I'm writing a story about a group of humans (or humanoid people) that are living on a spaceship that's orbiting a planet with high gravity; the force is too much for direct human involvement. There are limited technologies available to deal with the natural terrain of the planet, as it is incredibly hilly and have a lot of thick vegetation, and so expeditions to the planet are limited to accessible areas using vehicles to conduct experiments and general research.
So as a method to gain direct access to the natural land they take a native species and genetically modify it to be compatible to the human brain (removing extra limbs, altering it into a "bipedal" makeup, etc.). I call it a mech primarily because there is a space in the head where the "pilot" enters, is attached via needles in the spinal cord and advanced invasive EEG, and then put to sleep, limiting the functions to the mech instead of any conflicts between it and the pilot.
I want a valid reason as to why they couldn't just create a mechanic "mech" suit? Them developing a pilotable GMO that requires invasive brain surgery seems really steep, as far as cost/benefit. I have some ideas; their resources are limited, they want to interact with the environment in a more natural way, or something about the environment is especially corrosive or detrimental to the metal they have access to. But it feels wishy washy. At least to me.
(Also on a side note, could high gravity planets even create very large fauna/flora? Or is that unrealistic?)
Any advice would be wonderful! I'm also very flexible as to what I have currently, and any sorta ideas or suggestions you have story wise would also be interesting. The more hard-sciencey the better