Why do you think they always want 50:50?
If they want to be difficult they will be regardless of whether you were married or not - you said partner I don't think you are. Nothing in law counter-acts their pissy nature.
So sadly you will need to think very carefully on this. So firstly make sure that the time component really is 50:50 of the difficult parenting (ie not weekends and holidays). Ignoring any bleatings about shifts etc. Keep a very careful detailed diary including all lates, classes, clubs missed etc.
There will be child benefit - if there are an even number of children then the norm is to split equally. If odd numbers and you dont agree and there is a counter claim then ultimately the HMRC get to decide - so make sure everything (school, doctor, dentist, hospitals etc) names you as 1st contact. Keep records of appts and who took the DC.
You may get UC in your own right but probably not as you are full-time.
There are essentials eg school lunches /uniform. It is fine that the DC have different choices when it comes to this, so when they are with you, eg packed lunches might be the norm. If ex doesnt want to bother, then he needs to pay for school lunches. I found with my ex that my purchased school uniform would go to his and not come back until it was too small, yet my DC were never allowed to wear it. So I used to accept gratefully 2nd hand uniform from other parents and the 2nd hand uniform shop - so I always had enough. I didnt have to buy the branded uniform so hold out on that - blazers are expensive and they only need 1.
The tricky part is the optionals outside school. Eg after-school clubs, dance clubs, Scouts/Beavers etc, sports clubs. You often have to pay a terms subscription in advance and perhaps uniforms etc. I found that my ex wouldnt take our DC on his time. "Luckily" the clubs didnt require a regular commitment but my DC lost rhythm, didnt make friends etc. It was absolute sabotage. My barrister at the time said the court would consider that "parenting differences" and there was nothing to be done (despite the obvious impact to the DC). I doubt anything has improved in that regard. So do not pay in advance. Let him book and pay, take the DC and then you can attend as a parent watcher. Offer to volunteer! But definitely mine missed out. And you have only to see the numerous threads on here from organisers berating that parents cant agree and DC turning up alternative weeks etc to see that most people are not supportive at all.