Yep, I have finally beat Doom 2, 30 years later!
Difficulty: "Hurt Me Plenty" , no cheating.
Doom 1 was so easy in retrospect.
But back in 1994/1995 I could only get as far as Suburbs before losing interest. At that time, I was about to dive into the 90s strategy game craze.

You are not alone. In "Doom II" the levels are more complex and indeed after level 15 the interest begins to wane. I, too, after so many years, go through "Doom II" in portions, while I can complete "Doom I" all in one sitting.

I'd rather said: they are mostly big and empty. Despite being quite tricky in some places.

But... with new music in DOOM+DOOM II, I played almost all levels. Now I finished at 21, and catched 31 and 32 after 15.
It's been so long, but I still remember most of tricks from a childhood.

Old versions I played like this - from start to 11, and then 1-3 maps for one run. Now I play 10-12 levels for one run.

I can't say that the levels are empty, since I always play on Ultra-Violent difficulty.
But in "Doom II" the maps are made so that you can take several paths to the exit. And thanks to this, you can skip most of the level, but at the same time, this part is not necessary to complete, since it does not give anything except bonuses at the end. A striking example is level 18. At the same time, in "Doom I" the maps are more linear, and only in a few moments we can take shortcuts.

I speak about level architecture. Empty brownish low-detailed halls with hordes of monsters thrown into your face. Unlike DOOM I where style felt in the level design.

In this meaning - you are right! )
But in original "DOOM I" we have weaks too. Bossfight levels are sucks. I mean E2M8 and E3M8 - it's not challenging part of the game. And whole Episode 3 is much easier than Episode 2 for example.
Episode 4 for me is like separate game, it's played like "DOOM II" but without additional monsters and super shotgun.

Technically, it is. 4th episode was released about year after DOOM II. Even iD's crew roster had changed significantly to that moment.

Now play Plutonia Experiment and feel Doom 2 was a easy as Doom 1. :)

Congratulations!

Yep. Plutonia is more hardcore. Revenant is most famous enemy in this wad.