I think Progressive Extreme Metal is a good term because it leaves a lot of room for diversity.
"Progressive" implies unconventional elements such as incorporating many different styles into its fundamental genre, a different approach to structuring songs, odd instruments and effects etc.
Then "Extreme Metal" is an umbrella term for all forms of metal that aren't shit - I think this is the most accurate term for a such diverse band in that it isn't very specific but still implies what Opeth is about.
Isn't Opeth simply Progressive Death?
Their albums are unique and very different from each other and at all, what makes it a bit difficult to catagorize them all as one specific genre, but I think they're officially ascribed as Progressive Death Metal.
Orchid/Morningrise: Progressive forest blackened death metal
MAYH/Still Life: Romantic progressive death metal
Blackwater Park/Deliverance: Progressive death metal
Damnation: Sad melodic prog rock
Ghost Reveries/Watershed: !???