It’s not hard, but other levels take longer than that.

it seems you could spam your way through it if you’re stuck by just using every item on every hand icon.

I guess? But I also think that “harder” old games like this are usually harder because identifying what you can interact with or making the items interact with each other is cryptic in ways that are either due to limitations in the game design or pixel hunting. Right after The Last Door I played through Call of the Cthulhu: Shadow of the Comet. That’s Infogrames, not Sierra, but it’s from that era of gaming. I really enjoyed it, but nearly every time I had to throw in the towel and use a guide, it was because of some weird game mechanic that I couldn’t figure out. For example, items in your inventory can’t interact with each other like they can in The Last Door, so you to take them to your hotel room and put them both on the desk. I don’t know if the mere fact that you need to use the desk in your hotel is supposed to be a puzzle or not, but that’s not the kind of thing that it’s fun for me to have to figure out.
I concede that you can spam your way through The Last Door, but if you’re playing it like that, are you even trying to enjoy it? I loved it because it was like playing a weird fiction-horror story.