Obviously, this is to a large degree a matter of taste, but I am far from the only Russian speaker finding Solzhenitsyn's language highly stilted and artificial. The English translation you're quoting is much better.
Interestingly, same is very often said about Vonnegut's translations into Russian.
I am, however, listening to an audiobook version, so maybe it's different on the page.
https://www.e-reading.club/chapter.php/11816/25/Voiinovich_-...
'Invented words' is what made me think of it, I don't think it's much in evidence in GULAG but it definitely pop up in his fiction. In Voinovich's parodic telling, it's due to an excessive reliance on the 19th century 'Dahl's Explanatory Dictionary'. It's a perfectly pointed barb.
I think where GULAG succeeds is in exactly what he puts on the front page "an experiment in literary investigation". Everything from 'В круге первом' through 'Как нам обустроить Россию' and later gets progressively weirder and, well, to put it politely, not better.
"если первая крохотная капля правды разорвалась как психологическая бомба - что же будет в нашей стране, когда Правда обрушится водопадами? А -- обрушится, ведь не миновать."
It's true that there's some dry and boring material, but I can hardly blame the author for that: he's the last living source for so many of these stories. I can picture him sitting at his typewriter, trying in vain to resurrect the final, fading memories of the countless missing people who touched his life—nameless and nearly faceless, probably buried in some unmarked grave—while waiting for the KGB to burst in any minute and destroy his decade of work. How much more painful must the process of editing be when the things you cut are fragments of human lives that might as well have never existed once they leave the page?
Unfortunately, there's now a contingent of the Russian population that denounces Solzhenitsyn for reasons of pure nationalism. It makes me so sad when I see Solzhenitsyn mentioned in the Runet, because it's almost always extremely vitriolic...