Fall of Plaguestone: why is my group failing so hard? [Contains Spoilers]

I’m new to pathfinder and started it with my group on roll20 due to Corona. I have decade long gm experience in in games like DSA (Das Schwarze Auge) and Shadowrun but not for Dungeon Crawlers.

Long story short: We are playing the Fall of Plaguestone module and my group (Rogue, Sorcerer, Paladin and Priest) needed hard intervention from my part as not to die on multiple encounters. Since I still need to get a feel for the difficulty I’m not changing anything from the module and play it by the letter.

The group nearly died by: (contains spoilers)

Enemies are totally able to one-hit my players with a critical hit and I feel the only reason they survive at all are the mighty resurrection possibilities of the priest (the 3 action heal). I’m looking for advice how to make the adventure more fun by not obviously saving my group but also for other opinion on how difficult that module is.