I don’t want to sound like I’m supporting TWAT. He’s clearly a callous, selfish arsehole. However, I think there are two separate issues here, and conflating them could back fire.
The early drop-offs on Friday aren’t evidence that he’s not doing 50/50: he still has them for an equal amount of time (slightly more, if you factor in that he gets them sn hour or so earlier on a Wednesday). Threatening maintenance claims (I know that’s not the plan OP) if he won’t keep them till ten isn’t reasonable and could trigger him to suggest the same regarding his extra hour or so on a Wednesday (or worse, he could just refuse to honour the 7 am drop off, or insist on the same for Fridays).
The issue is the lack of sticking to the agreement, and the sensible thing to do, is just to say that it’s clearly not working so let’s stick to X time. I suspect that when he made the arrangement, he was busy convincing himself that he is a caring, involved father, rather than a shit who’s abandoning his family because he doesn’t like being a grown up, and he’s now decided that doing the school run every other Friday is enough, and that he’s currently agreed to do slightly more hours than OP (yes, we all know it’s his mum doing the bulk of it, but to him, that’s the same thing) so is backtracking. He doesn’t have the balls or maturity to discuss it with OP, so he’s pulling stupid stunts like turning up early unannounced. It is not being a mug, to recognise this and put a stop to it by saying, let’s change the time to 8. If the kids are up early anyway, it’s not unkind to hand over earlier.
The issue of true 50/50 parenting is still there , of course, but to be honest, I’d leave that for now. It’s in OP’s financial interest to be the main parent to ensure she’s the one who can receive the child benefit and any UC, so whilst I absolutely would be recording all examples of him not actively co-parenting and all expenditure, I wouldn’t be raising it with him for now (other than telling him what you’ve spent to give him the chance to do the decent thing and give you half).