said by Dennis:
Background: House built in 1989,...
I suspect that the pipe is just clogged up with grease/junk/i don't wanna know after the last 30 years and from the previous owners.
If the house was built in 1989 it does not have 30 years yet.
In any case I think you need to buy any enzyme liquid to cut the grease alternating with the snake. I hope it helps.