OP I would approach it from the point of view of what 50/50 would look like for him, and for the children.
That on his days he would be 100% responsible. All the childcare arrangements, all the parenting, all the admin, all the time juggling, cooking, cleaning, appointments, providing uniform and clothes and keeping them clean and well fitting, giving them quality attention, ferrying them to clubs, leaving work for a child with D&V and nursing them at home and all the clean up, etc. Make it clear you'd be doing none of that on "his" 50/50 time.
Are you having mediation sessions? If not I'd offer some so you can discuss this with a third party and have it all set out and try to come to an agreement. He needs to see the reality of it and know you wouldn't be picking up the slack on his days.
I would advise you not to do alternate weeks, but to have set days at handover times because you can then be very clear about which days and times you would both have to pay for childcare for example, or be on call for an ill child. Also makes childcare costs and arrangements easier to sort when you have set days rather than 1 week on, 1 off where you'd both have to agree on childcare and pay half each - I've seen exes not pay their half for example of decide they aren't happy with the childcare etc, it can be used as a tool to control or make things complicated.
The house and the maintenance etc, I would keep that very separate for now from the discussions about contact. The conversation about contact should be focused on what can he actually do around his job, and what does he truly desire to want to do as a parent.