Hello Alex,
I’m in Oklahoma. I can help you with items specific to a US installation, but I’ll defer the rest to Robert Wall.
The use in North America page has links to several sources of CTs that will fit 4/0 wire.
You may want to have a look at this article.
I’m very curious about your load center. I’ve never seen one with the bus bars exposed as they are in your picture.
Very unsafe situation you’ve got there unless your main breaker is switched off.