First, well done to both of you for trying to resolve it without solicitors. If you can achieve that, it's definitely the right thing to do.
I will offer you another perspective. And I'll start with a question, that's deliberately a bit challenging - why do you believe that two nights is "too long for them to be away from their mother", but 3-4 nights is absolutely fine for them to be away from their father? Do they not love their father too? Do they not have the same right to a close and meaningful relationship with their father, as with their mother? If you can put yourself into your children's shoes, you will see that is clearly not a reasonable position - you are both their parents. They love you both. In their eyes, and in the eyes of the law, you are equal parents.
It sounds as though there's an unspoken assumption in your position that your role as mother is somehow superior to that of their father. It also comes out in some of the other comments on here from other mothers, suggesting that men can't handle the day to day care of kids. That is simply untrue, and it is highly sexist prejudice. Men are often caring, nurturing, loving and very capable parents.
I've laboured that point, but it's important. If you can set that sexist baggage to one side, you'll be able to do a far better job of putting the needs of your kids first.
Your concern about quality versus non-quality time is a legitimate one. For me, an ideal split is one where both parents are closely involved with all aspects of their children's lives - whether that's homework and days off school sick, or fun & games at the weekend. That's because a really close relationship is forged from all that stuff. And there's a wealth of research that supports how important it is for kids'wellbeing, to have that quality of relationship with both their parents.
On that note, there's a lot to be said for 50/50 equal shared care. That's the set up I have - two weekday nights with each parent, and alternating Friday to Sunday nights. But it really does depend upon whether that is practical - that only works when both parents can commit to being there for the kids.
I totally get where your ex is coming from with his suggestion - he doesn't want to go for long periods without seeing his kids. I also get why you don't want to lose so much weekend time. So the key is compromise. Can you offer more time during the week, in order to secure the full weekend every other week? That could address both your concerns.