if you want to leave the EU, I'm going to pretend that you want to get rid of it completely
No, that's a fact.
If the UK leaves the EU, but in addition to that says it is not going to be a part of the SM, the CU, the ECJ, and the ECHR (red lines), the consequences are that the UK becomes a third country. This means it gets practically nothing from the EU.
There is a legal framework to be inside and out out of things. There is no legal framework to be outside and opt into things.