Have you considered going for a shared residence order with equitable time spent with both mum and dad?
My partner had a truly excellent arrangement with his ex where his 3-4yo daughter spent 6 nights with him and 8 nights with mother. His daughter absolutely thrived on it. She had a clear understanding of having a home with mummy and a home with daddy.
This was working brilliantly until for reasons known to his ex she stated messing around with the arrangement. It ended up going through the courts initiated by his ex... After 8 months of hell he did get shared residency but only after alot of damage was done to the good working relationship they had had, both their bank balances (the lawyers were the winners here folks) and damage of the daughters relationship with the mother (shes angry with mum for stopping her seeing dad as much)
You guys look like you could work really well together for the benefit of both the kids. Your ex has already agreed he was wrong on how he treated dd and is going some way to rectify that yeah? So if you work together for for the best interests of both siblings you can carry on doing that in the future.
People may trot out the one size fits all scenario of weekend contact with dad... but one size does not fit all, children, separated parents and circumstance are all very individual!
Im currently 38 weeks pregnant so my partners lil girl will soon have a new baby sis- something shes very very excited about! Yes I can foresee difficulties in the future when for example partners dd wonders why her sis gets to be with daddy all the time or why they cant go to the same school but we will endeavor to deal with any and all problems as they arise with the firm goal of doing what is best for both sisters.
Good Luck allgone- hope things work out for everyone!
Remember keep talking and try to keep the lawyers out of your lives, they are a sure fire way to make things a lot lot worse IME!