Here's my answer as someone who was agnostic when younger, then a "non-denominational indifferentist Christian", then Catholic:

In a mixed marriage, I thought the non-Catholic must agree to let the Catholic parent raise the children as Catholics until adulthood as a condition which must be upheld for the Church to approve the marriage.

So, if you are already planning on not having them stick to Catholicism, you will make your promise to the Church and your wife a lie. It's good to not pretend to be a Catholic to hold up a facade, but you already might be starting out with a lie, anyway. If your wife knows you are breaking the promise you made, and if she sees you slowly loosening up about your promises and maybe even enticing your kids, it's going to hurt your marriage.

If I am wrong, and only your wife had to promise to raise your children Catholic, then what you have said already implies that you have little to no respect for your wife's promise.

If you want her to be happy well into future decades, don't marry her. Instead, let her find someone who will not cause conflict in her childrens' minds and lives, because no matter how nice you make it out to be, you are already, tacitly, showing yourself to be a sort of passive adversary. You think it's neat that she's good at this "religion game", but you'd be so happy if she were an atheist instead! Well, what's likely is that, like all married people, you will both become intimately aware of each others' faults more than you ever imagined now, and with that familiarity you certainly may not think like you do now. And, you might decide to give up being Mr. Passive Atheist, hoping the kids will see it your way. Instead, there's a good chance you will say, enough of this Catholicism stuff, our kids should not be led into religion.

Then, either you will be in for years of fights, possibly quite bitter fights that will lead to divorce - and you have no actual respect for your wife being Catholic if you are going to gamble with possible divorce with her. She may never be able to marry as a Catholic again if that happens. Sure, annulments may be too easy to get these days, but that may not always be true. You would be consigning your ex-wife to either apostasy if she re-marries after divorce, or she will have to live the rest of her life single.

Or, if she has weak faith, you might not be in for a lot of fights, or they may be easy. Instead, years from now you may find you are able to win the arguments and get her to not stick with her promise to the Church. You may even get her to become a non-practicing Catholic, or agnostic, or even an atheist. But then, you will have made everything in your original post here a lie, as well as everything you promised to her at the beginning. She will always remember.

There are so many things that are going to happen between now and when you two have your kids, you need to do a lot of very hard thinking. You have a very rosy picture of things and you are partially blinded by love.

Let me, a Catholic, tell you, an atheist, something very interesting - feelings aren't very important. The feeling of love is great, but the actions of love are what give it real meaning. If you are willing to lie or capitulate just to keep that feeling of love you have now, and if your actions come from and are dependent on your feelings, you are in for serious trouble. Feeling is a weak basis for anything with the exception of art.

If you want to pursue feelings over principles, then I and many others would say you have little business being married. If you want to be realistic, you have to plan for times when you might be unhappy with life, or with her, and you might not feel like putting up with "the Catholic stuff". You might go through stuff that makes you unwilling or unable to hold that up. You have to put down the good feelings for a bit and be honest with yourself and your wife, and talk about what you really think of religion and what you really plan to do about your kids being raised Catholic. If you don't, you're letting yourself and your wife fool each other by buying into a facade. You're already weighting things in your own favor

You're already saying you're fine if the kids abandon their faith, so how readily do you think anyone would believe you won't entice them or drop statements here and there contrary to what your wife and the Church says? Are you really going to keep thinking this way 18 to 20 years into the future, or does it just sound good right now? It may get you over one hurdle, but you cannot avoid the hurdles in the future.

I'm very curious how old you are. If you and your girlfriend are in your early 20's, you need to slow things down and do some hard thinking. If you're older, well, same thing.

P.S. I agree with ThereWasADream's comment - that letting your kids decide at high school age whether or not they want to go to Mass is the worst time to do it. Teenagers do not have adult minds, and we all know from personal experience that during the teenage years we basically try and un-discipline ourselves because we are not mature. For you to go easy on them during those years makes all your accommodations and stated intentions into meaningless platitudes. You are basically saying you are just going to wait for the right time to sabotage what your wife has promised the Church she will do. You are waiting for one of your kids' most vulnerable times - when they will think like they are adults while they are still just kids - to woo them into complacency away from what you see as your wife's neat but meaningless promise.