I don't know. I'm usually ready to watch a meandering film with slow pacing, but I know people who won't put up with that, so I might like movies you find unwatchable. In contrast, there's a subset of French slice-of-life movies that I just generally dislike while other people love them. Anyway, I've only seen 4 Tarkovsky films, and they all move more slowly than, say, a superhero blockbuster. Of those 4, I guess the most accessible is Solaris. Some days/weeks later, I'd watch Andrei Rublev as a change of pace, then after another wait try Stalker.

Those should be enough to give you a feel for the director, and then maybe you can try Mirror and tell me what I'm missing. I feel like a bunch of symbolism is going over my head. Are those Maoists shaking little red books? What does that evoke from a Russian perspective?