New drain field is okay. But that is not the problem
Toilet flushes okay, but with large poop it goes down slowly
After it breaks down the toilet flushes okay
Problem seems to be between the toilet and the outside.
Clean out plug shows water going out. This partial block
May be a tree root or something that hold up large items
Until they breakdown. Small items or loose material has no problem.
Guess need a camera to see what the blockage is??