I’ll second Neil, and say that Keller sensor are very high quality and work really well with Mayfly RS485 integrations. I’ve helped deploy many AccuLevel sensors.
There’s a small chance that Keller 36XiW-CTD uses the same Modbus map as the Acculevel, but I think it is unlikely. You’ll need to get the Modbus documentation from Keller for that. I found that Keller’s tech support staff was super helpful.
The place to define the Modbus map for this sensor andstart testing is here: https://github.com/EnviroDIY/KellerModbus. That’s the library that ModularSensors uses.
SDI-12 has the benefit that it uses universal commands, doesn’t require a special “map”, and can generally be implemented on the Mayfly without a special adapter board. The drawback of SDI-12 is that it has a much less stable communication protocol (both logical and physical), and often the sensors are more expensive to purchase that Modbus options.