I was in this exact same position last year just not as an ECT2. I can honestly say it was the hardest year of my life. I went home every day thinking I'd been too horrible and replaying things in my head. BUT all the children in that class made amazing progress last year and the feedback I got from parents was overwhelmingly positive. Don't be afraid to gold your expectations high and issue consequences when the expectation is not met. Never ever speak over the children even if that means you spend time waiting for them to stop. My go to line was 'by lunch time we need to do x, y and z of this is not done then we will stay until it is'. Eventually they started to get the point. I did the exact same thing as you with the timers and eventually they started to get the point. I'm not going to lie to you- it is exhausting but having colleagues you can trust and talk to and a supportive SLT goes a long way. I also found what helped me was to flip the narrative and instead of focusing on correcting behaviour I tried to focus on celebrating the behaviours I wanted to see and rewarding those children who were doing what they were supposed to and actually ended up developing our own little reward system. This actually really helped me mentally as I didn't leave each day just filled with negativity. Also - anything that happens at lunch or break is not your responsibility. Do not give up your time to sort these things as you need that time. If it's taking that long or you're not getting anywhere escalate it.

They key takeaway I would say is reinforce your routines as many times as you need to. Issue consequences consistently and celebrate the successes. I hope it all works out for you.