Shelly have a few more modules, have you taken a look at this one? The PRO 4PM
If your replacing the modules in your current consumer box, there are regulations on what you personally can do. Here in the UK it’s a no no all together. You have to get an electrician in so they can provide a safety report and certificate.
Personally I wouldn’t aim for the cheapest option when it comes to your house electrical supply.
What’s wrong with WB-MR6C? Price/relay looks fine, doesn’t it? Ethernet to Modbus converter can be had for 25 USD. ( 3 x 60 + 25 ) USD / 18 relay = 11,40 USD/relay - I doubt you will find anything cheaper.
I am not looking for the cheapest solution but a reliable and fair price solution.
I am building a house where I´m planning the electrical installation and I would like to have full control of installation by using Openhab or similar platform.
I have already done a few projects where I have used Siemens LOGO8 or Micro 800 from Rockwell Automation but on both systems I had to install extra relays to protect the output modules because they have very low Switching capacity.