First, I'm sorry to hear of your fertility issues. If finances are really holding you back, run the numbers as I can't see it being another 10 years of working if you have kids. But often I think people say finances when really it's a little finances and mostly something else.
Re Christmas & birthdays: Just because you guys started off giving large gifts, doesn't mean you have to continue. You have some choices such as:
1) Simply start giving smaller amounts
2) Simply start giving gifts rather than money so it's a bit less obvious (you can also transition with some handmade gifts to make it less obvious)
3) Transition to giving family gifts rather than kid gifts (e.g. a set of play golf club clubs, a board game that all the kids will be able to use together)
4) Upfront say with a laugh that their fertility has busted your budget (not these exact words of course, but phrase it that it's taken with an understanding smile) and you are going to need to cut back
Honestly, very few of of our friends give our kids gifts. The one that does gives either old items they are done with (e.g a play shopping cart) or small $10 items. It's a token. Particularly when they are small the kids won't register it.
Re the friends asking: Next time one does, quietly say that it hurts you that they keep asking, when they know your wife has endometriosis. Or for a quieter approach, pull someone aside to tell them that and ask them to spread the word to the others and ask them to stop asking. This is easy, because I bet they are just being thoughtless rather than intentionally hurtful.
Meals is a little tougher if your friends haven't already brought it up. If they have, great, you just accept their offer to pay more. Easy peasy. We've had friends off (pre-kids for us) and we've done things like they take care of the tip to even it up rather than try to exactly figure out bills. (This approach is something we've also done with a non-drinker friend, to even it up so we paid our fair share when ordering drinks.) This makes it easy on servers too. Our friends generally started offering more once the kids got to 5+ so this may resolve itself as people start being aware they are more costly. If they don't, you can just say "looks like our portion is X, does that sound right to you?" Or ask for split checks. Note that we never bothered to do this with family though, as we figured that my sister hosted us more often than we hosted her, so we weren't going to nitpick the occasional restaurant meal out.