Just downloaded that octopus. The slope at the base of the model is a lot steeper than appeared in the photos here, so I think we can remove the concern about edges of the PLA lifting.

The next thing I'd do it take a look at the base of the prints that are lifting. Do they have any sign that they are lifting at the edges first (usually there's a visible 'crease' where the outer edge has lifted while the centre holds). That will give an indication of whether it's mostly bed adhesion which is the problem, or the print leveraging itself off the bed. You can also do this by watching the first few layers as they print.