It seems silly to get Christmas lists and large numbers of gifts for adults still. They all have income, unlike kids who need to wait for special occasions to get things they want.
We currently trying to negotiate this - late teen one at Uni. Uni child didn't come last Christmas - did weekend before - but might this - which wasn't immediately clear so she got stuff to take back and then additional food.
Last year I did "something you want, something you need, something to wear, something to read" for them, which worked well
I think that helped us cut back for eldest birthday.
It is tricky because my parents went to money - always welcome but more than I can give - and IL were all over the place till GC came along to buy for.
So get what you mean about needing to evolve it.
Many other family traditions we won't do when youngest leaves home - Halloween, bonfire night won't mark at all and what we do has changed with time their ages and our locations- though when at uni will still send gifts along those lines and easter will pro just be an egg each - not sure will want to do Christmas decorations either - though if our parents start to come over might though they say they haven't bothered in their homes for years with decorations.
My IL did holidays with DH abroad with DH late teens early 20s till got too expensive - my parents had us back till we married or settled down not sure my parents really enjoyed that though - liked seeing us but not wild about Christmas any more but made no changes.
I wouldn't do Christmas at all if it wasn't for them. A few years ago one of the middle ones was in hospital having surgery over Christmas. The next they weren't there at all and never will be again. But I don't think it's about that.
I'm really sorry. I do suspect this might be playing into it all as well though even if it's just subconsciously.
Maybe theater/church/cinema trips or meal out- something you weren't doing before but can do together.