We have incredibly similar circumstances, stayed living together for 18 months after split (without telling the children) due to my career change, and on hindsight regret it immensely!
Mine are 6 and 3, and we do 5/2/2/5. We both worried 5 days may be too long, but they got used to it pretty quickly actually, and I often found that they did better on the longer stint as they had more time to adapt.
Please don't muddy the waters with doing too much together. We are, on the surface of it, amicable, and will do things together, but I am mindful of it not crossing into confusing, and you also need to think about what is sustainable down the line should one or both of you enter into a serious relationship, you don't want to be in a position where you've committed to spending 4 nights a week with your ex husband and then having to tell your new partner he has to leave the house because ex won't tolerate it, or never being able to spend time with new partner because ex won't allow him in the house but you've already signed up to 15 years of Christmases together, for example. It's easy for DH to want all these things when he hasn't got anyone/thing in the picture to jeopardise it, but as and when he does and things have to change it'll be very confusing to the kids to understand why it all stopped. don't agree to anything you don't think you could tolerate regardless of who/what came along (eg a few hours together during a handover period at Xmas is one thing, spending 3 days together for Xmas eve/day/boxing day is another if there's other partners/families to consider!
It's tough in the short term, because you obviously want to be included/part of as much as possible (and vice versa), but it won't help the children. Since the split 6 months ago, we went to fireworks together, trick or treating, one dinner out as we'd both attended the same appt for DS as wanted to give our input, and then we have agreed to continue our day out to a local Xmas tradition each year as neither of us wanted to give it up, and it's easier with 2 adults tbh!
The benefit to 5/2/2/5 is (imo) that the weekdays are fixed so it makes for easier planning, and indeed our 6 year old has identified quickly which days are where, and therefore who she'll be with, if she didn't know whether her dad was going to rock up to do bedtime, for example,it would undermine that, and lead to pushback on the times he wasn't as she wouldn't understand why sometimes he was and sometimes he wasn't. I feel like it gives you the worst of both worlds as well, not with your kids full time, but still not able to make plans for your free time, and still having to consider someone else on your parenting time!