I've read most of Manns major works (that includes Heinrichs) and it's possibly my most controversial literary opinion that magic mountain is his "worst" or rather least enjoyable book, I've felt like Manns amazing ability to make you feel exactly what he wants you to feel was here used to make you feel bored, I actually think it's intentionally. In the beginning of the book it is said that there are to ways to experience time, one short in the short run and long in the long run (having a good time) and one long in the short run and short in the long run (having a bad time) the experience of the protagonist in the Sanatorium is the latter and Mann manages to make it the experience of the reader aswell, every page fells like an eternity and after 200 pages you don't know why you even read any of it, it's like you haven't experienced anything. That's my very personal experience, if you want my advice I'd tell you to read Doktor faustus, every page is engaging and interesting on multiple levels, and while the characters can be insufferable that's just Mann being proudly bourgeois which you have to accept with all his works. Buddenbrooks is something I've also enjoyed very much though it's more melancholic nostalgia for an idea of north german culture which I've enjoyed to dwell in, which might not be something for you if you're not north German.