Plumber won’t complete snags

I got a lot of plumbing work completed in May/June 2026 including new bathrooms and a new heating system. Since then we’ve had issues, two showers are broken, hot water is intermittent, the immersion isn’t wired and three radiators are not working. He also has some tidying up to do in the wet rooms.
Unfortunately I paid the final instalment to him as he was working here at the time, and two days later without checking the system he said the job was finished and off he went. I told him about the snags I knew of at that time and he said he’d be back. That was almost three months ago. Lesson learned of course!
He now won’t answer calls, texts or voice notes. Any idea where I go to from here? A registered letter perhaps? Total job cost €36,000. Don’t come down on me too harsh this is my first time dealing with tradespeople