OP I'd drop them, for the simple reason that they've already dropped you.
I agree with you, if they valued you at all they'd be making the occasional effort - and they're not. That speaks volumes.
Even if you were in the same phase of life and had DC yourself, they'd still be in the exact same situations they're in now, "unable" to compromise and make any effort at all for you. So I'm sorry, but I'd consider these friendships are dead. If you had DC too you'd either still be doing all the running or the friendship would die out. If you make the effort now I don't think they'll do the same for you if the situation is reversed in the future and you're the one with young DC, they'll just have some other excuses for why they "can't" make any effort to come see you. So if you continue as you are you need to do it without expectations of having the favour returned or staying friends for life.
I've noticed its often the way that when someone drops someone else it doesn't necessarily look like being busy and refusing to meet up at all, it's often seems to be either lots of last minute cancellations or this thing you're experiencing, where they're only happy to meet if it's low cost/low effort on their part or an activity they really want to do, because the person they've dropped is just an option, something to kill time with if they happen to have the time/energy and are bored, a "low-key" (as another poster said) non-priority. My advice is to never prioritise people who only consider you to be an option.
People are saying your circumstances with losses aren't comparable but they are. You're repeatedly making the effort to do things you find very difficult so that your friends feel valued. They're not doing the same for you, they don't really care how you feel or at least not enough to make any effort for you, it's all about their family now.
I'd pull back drastically, see them once or twice a year, stop with the gift buying and other upsetting activities, then see if they make the effort to step into the gap you've created, to keep the friendship going. If they do you can always regain closeness and make a bit more of an effort again.