We realise he doesn't have to pay for the "extras" but he doesn't want the DSC to have to stop doing clubs etc
Which is probably true, but not what you said up thread when you defended your DPs choice to continue paying the extras, as well as the CSA amount, saying that you manage, and that you consider it unacceptable for your DSC to miss out.
You seem to have tied yourself in knots a bit. I'm also confused about your court action regarding your Ex, which you have posted about extensively and which previously, you have explained is a Section 7 (Children's Act) case. That contradicts what you have said in this thread. Wasn't there some issue with midweek contact that you wanted to sort out?
You have created what professionals would refer to as a chaotic lifestyle for the DCs in your care. Their ages indicate that the split between their parents has been relatively recent (either you or your DP has a DC under 5), since then they have moved homes, have a new sibling, have had a number if different primary carers and care arrangements (btw, who did care for your DSC while your DP worked before you took over?), there is tension between both you and your DP and your respective exes, you are both involved in ongoing legal action, and there is financial pressure on the household. All that against a backdrop of a 50:50 care arrangement which is well documented to not be in the best interests of young DCs. (And yes, I have researched it, given that I agreed it for my own DD).
You may be able to cope with all that. The DCs may not even be showing any signs of being affected right now. But have you stopped to think whether there may be hidden, or long term effects? Have you sought any kind of professional advice to best provide for the DCs that you have chosen to place in an incredibly unusual situation?
Or are you so certain that you are right that your confidence extends to the DCs and you know that they will be fine? Very few parents can claim to be certain - I almost envy you your peace of mind.