As well as being concerned about firmware and software I would give serious consideration to hardware. Industrial devices are "hardened" to abuse such as transients on the supply, inputs and outputs. Inputs and outputs - including analog - are usually electrically isolated from the CPU and logic so that no fault currents go through the CPU, etc. A modular approach allows replacement of damaged I/O modules without having to reprogram or disturb the CPU.
Figure 1. An industrial Pi. Source: Linux Gizmos.
The device of Figure 1 was picked at random from a Google image search. A few features suggest that it might be suitable: A separate "hat" I/O board, removable rewireable terminals. Things I expect to see - but don't - include a line of opto-isolators for the inputs and opto-isolators or relays for the outputs. The black object on the left appears to be a coil or transformer which suggests a voltage converter - possibly with isolation - to power the inputs and outputs.
Industry standard is 24 V DC for device supply and I/O. Analog interfaces are usually 0 - 10 V or 4 - 20 mA.
Figure 2. An industrial DIN rail mounting case to finish the device off.
The problem with this approach is that you now have a one-off system that only you can support. Buying the hat and the case probably brings you above the cost of a micro-PLC which has all these features built in. If the system packs up while you're on holiday who else can support it.
The commercial option:
Figure 3. Micro-PLCs are available in this format from all the big players including Siemens, Allen-Bradley, Mitsubishi, etc.
Many of the micro-PLCs now feature Ethernet ports. They are available with / without built in message / programming displays and keypads, can be front-panel mounted and are well priced. Check the cost of programming software. It might be a consideration.