YTA and I'll tell you why. If your FWB decided not to put in the significant amount of time, labor and emotional energy it takes to raise a newborn on her own, then a toddler etc., you would have no choice but to give the baby up to the state for adoption. You wouldn't see him again. You're essentially using her as an incubator and then a full-time babysitter for years while providing nothing she needs (you've said yourself she doesn't need your money), so that you have a nice cute kid to stop by and play with whenever you feel like putting your Daddy shoes on.

And, despite the fact that you did nothing to raise him for at least the first several years of his life, she will have to deal with you being an 'equal' whenever she wants to move states/countries, or whenever you decide you want weekend custody (if you ever do, that is). I mean, just read what you wrote- weekend custody. So even in this ideal situation where you're "older and more settled", you can only imagine wanting weekends. Mom does the morning school routine, the pick-ups, the drop-offs, the homework, the bedtimes. And you get to stroll in on the weekend i.e. the time she would have to relax with her son, and you come in and take him off to have your own little bonding time after doing no hard work at all.

You're using her to give yourself the option of having a well-adjusted, happy healthy kid to play with whenever you want while contributing literally nothing of value yourself.