You are getting ahead of yourself.
Don't try to change how things are right now - it is a waste of time. Ignore him as much as possible, don't try to force him to share responsibility.
The calendar isn't working, so drop it.
Just go full speed ahead with the divorce.
so now I'm worrying, if it's like this whole married - how will it worn when we separate?
Have absolutely fixed times for hand-overs of the kids, set in stone (if not written down on a court order). Don't try to 'be flexible' or 'take it as it comes'.
You do the drop-offs to his place, so that there is no possibility of him dicking around by being late or early and messing with your plans.
He does the drop-offs back to your place. Arrange to be ready for them well in advance and don't make plans for after in case he is hours late back.
When they are at his place, they are fully his responsibility. e.g. if DC is due to go to a birthday party on 'his' Saturday, you have nothing to do with it. Hand him the invitation, or forward it, and that is the end of your involvement.
Don't step in to rescue the situation when he messes it up.
Likewise, at his place he has to have DC clothes available and do laundry - don't send the DC with spare clothes or have them bring back laundry.
if/when our marriage ends we will need to agree shared responsibility with kids - he ignored me.
You cannot force him to step up and be a parent.
Beware of him agreeing to X number of nights to minimise maintenance, but then still dumping the maximum amount of work on you, by constantly pushing boundaries, accusing you of being 'inflexible', etc. e.g. dropping them back straight after breakfast when he is meant to have them until the afternoon, or asking if you can take them for extra nights as a favour, without the maintenance being adjusted.