I’m sorry I’ve missed this thread until now.
I agree that when interfacing with a 12V powered device through the RS485 adapter, there shouldn’t be much risk of frying the Mayfly. I would still go with a separate power boost instead of stripping the resistors on the Mayfly, though.
I would *NOT* recommend any RS485-to-TTL adapters based on the MAX485/MAX3485 chip. Getting the timing right for the enables is *really* tricky and I haven’t been able to get it to work successfully at all. I have bought a couple of boards with automatic flow control (looking similar to the one Calvin pictured) and all of them have worked.
WRT Modbus communication, I have not tried to interface with In-situ probes yet, but in working with other sensors I found the ModbusMaster library to be really confusing. I ended up writing my own library to handle modbus interfacing with sensors. It’s specifically written with someone who knows very little about modbus in mind. That library is on GitHub if you’re interested: https://github.com/EnviroDIY/SensorModbusMaster