I can relate to this. I (guildless, mostly solo player) got excited about Thorgast because it sounded like they were aiming it precisely at my playstyle. However, it turned out flawed because:
(i) for my class/spec, most of a Thorgast run is bereft of challenge and thus extremely boring; and
(ii) the only challenge there is comes from the final boss, but reaching such boss requires a long time invested in the boring part and, on top of that, the fight itself often turns out to be an RNG fight instead of a challenge to overcome through planing and skill; and
(iii) after having upgraded my legendary of choice there is no meaningful reward to pursue in there.
Not even 35 anima
Twisted Corridors is similar, but worse (boring part takes much, much longer, more punishing boss RNG). I made the mistake to waste 2.5 hours in there once, only to be crushed by the level 16 boss. I wasted one try getting crushed. Then I wasted two tries examining the boss's spells to figure out what was killing me. Then I spent two tries trying out combinations of "oh shit" buttons / dps cooldowns. Both failed, and although I had more ideas to try (and was prepared to wipe in there for a much longer time), there is no way whatsoever that I will spend another boring 2.5 hours of my time getting back to level 16.
It is in fact more fun to try my hand at some Legion hc/mythic bosses. If I wipe on them, I can change my tactic and try again as many times as I want without getting locked out for the week by RNG. If I manage to kill one, there's a chance of a transmog reward. In other words, Thorgast (in its current incarnation) is less fun and less rewarding than soloing old raids.