Financials help when son lives with me full time

My son's mum seems to really struggle with our 9yo son at times and it's becoming more and more. I try my best to support her explain to our son that his behaviour at hers isn't acceptable, he's not perfect at mine all the time, although I seem to be able to manage this and can talk to him and calm down. I think sometimes he struggles with disappointment, he's definitely seem to be getting better behaved at mine. I think sometimes he plays up as he knows there's a chance of coming to mine, But at the same time I won't discipline him for things I can't see the whole story with when at hers I want him to open up and be able to talk about things which he generally does.

She has dropped him off to me multiple times when he's misbehaved, and threatend that I will have him full time on more than a handful of times, I won't say no to this, I've been able to so far manage the suprise of this with work, at very short notice.

Our arrangements is very much 50/50, though he's usually with me more. She claims child benefit(probably wrongly). No maintenance between us.

I think there's increasingly possibility that he will be living with me full time before long. If and when this happens I want my financials in order, and am completely unaware of what help if any I may receive.

I work on average 39hr week, Mon to Friday. If he was full time with me then that will have to be 22.5hrs with the use of breakfast club. And 27.5hours if I used after-school clubs but the cost of these will outweigh the benefits of working 1 more hour and they're sometimes not on, so not 100% reliable.

I'm currently earning before tax £37500, but if I was to drop to 22.5hrs I'd earn approximately £21600 a difference of £14800.
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I'd claim child benefit and
child maintenance- though I think this would be nigh on impossible to get from her. Nor can I even guess what her earnings are.

I have a 80k mortgage left. And savings over 16k. With this in mind I doubt I'd receive any support.
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I Don't really have anyone near enough to help with school runs either.

Probably in around 12/18 months he could start walking home. But I'd still want to be home so working longer may not be an option.

​​Any advice and help please if my son does indeed live with me full time.
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