I have 2 nieces and 3 nephews. The oldest one is 20 years older than the youngest one.
I used to give them gifts at Christmas and they would get a small gift for me. By the time the 3 oldest were married and having their children, I had decided to only give gifts to the little kids as I do see them at Christmas. I spoke to the 3 oldest and told them and also that I expected no gift from them at all. The 2 younger ones (children of my youngest sibling) were still kids at the time so I set age 30 as the cut off age for them.
This has worked out well. I only buy birthday gifts for the 2 youngest and Christmas presents for them and for the children of the other nieces and nephews. I still see them all around Christmas time and I like buying gifts for the kids.
However, OP, my family has never spent the kind of money you are talking about for birthday or Christmas gifts. That would be way beyond what I could afford and what anyone would expect.
Actually, when my mother died, we all discussed how we would proceed with gift giving at Christmas and cut back accordingly.
My younger sister's husband comes from a big family and they get together at Christmas. They have a name drawing around Thanksgiving. One "pool" for the adults (my brother-in-law and his siblings and their spouses) and one for the kids. Each person gets a single gift from the "pool". It would cost a fortune to buy for everyone and this way there is a limit of how much is to be spent on any gift.
As for thank yous - sometimes I do get a hand written card from the little kids which is nice, but once they hit their teens, etc, it becomes less frequent. However, I'm pretty close to my relatives so I don't care much.
I almost never got gifts from my aunts and uncles and only a small token gift if we happened to see them at Christmas, so I don't think anyone is all that upset at the reorganization of gift giving.
A couple of suggestions, OP. Starting at some point, possibly at the start of the year to be fair to all, cut back across the board how much you send. If questioned, tell them your financial advisor told you to cut your expenses.
Of course, you can use the passive aggressive approach I saw mentioned in an advice column. Grandparents wrote in to complain about not receiving thanks for checks sent for birthdays, etc. Dear Abby (or whoever) suggested not signing the check. Guaranteed to get a response. But them, so many people don't use checks at all.
Or you could give them gift cards to stores they don't use. That's a gift, but pretty useless. But, then you are still spending your money.