OP, I understand your upset and frustration, I’ve been there myself. I tried everything from lettting DS do what he liked more or less to being super strict. In the end I went on a parenting course run by the LA. Could you see if any of these are available in your area? It might be hard at the moment, but if you can get yourself on a waiting list it’d be really useful for tracing you coping strategies.
The thing I have found most helpful is to include DS in deciding what punishments he will get if he plays up.
take the pinching the tablet for eg. Me and DS would sit down when everybody was calm and I would tell him that I thought his behaviour was unacceptable, why, and how it made me feel, then I would ask him how he felt about it, whether he thought it was right etc. There might be disagreements but actually when you get them in a calm mood and address it this way they tend to come to the understanding themselves. Then we discussed what we thought the punishment should be the next time he did it. I said what I proposed and asked him what he proposed, whether he thought it was fair etc. Then we came up with a plan, so, if you pinch your tablet after bedtime we have agreed that you won’t have it the next day for example. Then I wrote it down and we both signed it, all official like.
I found this really worked because DS had input into it, we discussed it thoroughly so we both understood each other and we were both clear and in agreement about what the consequences would be.
It might be worth taking this approach with your DC. You’d be surprised how successful it can be. Basically you’re giving constant reinforcement, rather than the damaging intermittent reenforcement that his father gives him. I’m not just talking about rules here, I’m talking about expectations, making sure they know what will happen and why, and staying consistent. Kids will always push at boundaries, that’s what they do, but the fact that there are clear boundaries in place that they understand is what gives them their security.
I do sympathise with you OP, I too had all the anger and frustration taken out on me and it’s awful. You feel like you’re getting the blame and being used as a punching bag.
Good luck, and do consider the parenting course, they’re really helpful.