You need to remember that you can only control your behaviour - not other people's.
Your friend doesn't want to leave her family and home and travel to visit you. Do you want to travel to visit your friend (with or without your children)? If you do want to then do. If you don't want to then don't. It really is that simple - if you want to and you enjoy it then do it. If it is a massive drag, a hassle to pack and be away or travel with small children and you would rather be at home then don't. You don't have to martyr yourself.
You know that your friend is not overly generous with cash. You know that if you lend her money she is slow to pay you back. Easy solutions to those things is match her energy and don't be overly generous with cash towards her. If you do something together get the cash upfront or don't do it.
What money her parents have, whether she has a mortgage, whether she works or how much her engagement ring cost is neither here nor there and none of your business.
You have sneered at her giving you 30 odd quid for your wedding. You have gone on to say that some people bought you sentimental things or nothing and that was fine. You clearly didn't think it was fine for this lady though despite what you say as you are using it as a stick to beat her with. It's neither here nor there. Who knows she could be so rich that she could give you any amount without caring and was just symbolically sticking some cash in an envelope, or it could be that despite wealthy parents and a high earning husband she doesn't have lots of her own cash, or it could be that she just felt that was what she wanted to give - what does it matter?
I find it interesting that you mentioned the "poorer" person you know. You gave them their wedding cash gift back and don't mind paying for their snacks/just want their company. The fact you mentioned this makes me think you enjoy being their more wealthy/generous bestower. I think you should consider that you do care about money and your friends financial circumstances and consider whether you resent those with more and enjoy when people have less - even subconsciously - and then if you don't like that about yourself you can challenge it.
I've seen a few different threads on here over the years of people that subsidise their friends activities and holidays only to find that one day they buy a flash car outright or pay for their children to go to university without taking loans etc etc.
Those people that were subbing their friends are then full of resentment. I would say in general if your friend doesn't want to go for a walk with you and buy her own snack it is more likely to be because she has other priorities than because she can't lay her hands on £3.50. If you want to pay because your friend would rather not pay for food but you want to go to the cafe then recognise that this is what you are doing - getting to do what you want by paying- rather than helping your friend.
If you don't want to travel to see your friend you don't have to set fire to the friendship. You could just keep in touch by phone/Whatsapp for a few years while in the thick of it with young children.