I have a project that needs moving from TRL 4‒5 to TRL 6‒7 in the space of about four weeks. This is tight but doable.
What is needed is work on:
A PCB that has sockets for GNSS, radio, Ethernet and MCU modules. It routes traces amongst the modules and provides DC/DC conversion and, crucially, RS-485 capabilities. A version of this already exists, of debatable quality.
A second PCB the function of which is to receive RS-485 data (× 6 lines), feed it into an MCU (COTS module dropped into a pair of 0.1" headers) and out via Ethernet (COTS module, SPI driven). This board communicates with up to six of the first board, at ≥38400 baud over up to 1,000 m of cable (but typically ~300 m).
Assume that there is no common ground between the two RS-485 endpoints, so you’ll need to be comfortable with the added complexity / know of a good choice of module.
The software to be used needs to be KiCAD 5.1 or newer. Some familiarity with FreeCAD would not go entirely amiss.
If you have experience in PCB design, especially automotive-grade stuff, and might be interested please get in touch. Given a few more details I would expect you to estimate a level of effort and expected duration, and quote either a price for the project or an hourly rate.