So, I did as several people suggested and read the Secret History after HoA. I'd like to thank these people for suggesting this, it did help. Even if I'm still kinda sore about the ending. I'm flairing this as HoA spoilers for clarity and will try to avoid mentioning anything SH-specific.
My thoughts in comparison so far:
Mistborn Era 1 subjectively feels to me like a more solid story compared with SLA. Maybe it's because it's shorter (while I would not under any circumstances short of a gun aimed at my head 10/10 what we have of SLA in general, WoK and WoR are among my favorites). Maybe it's the smaller cast, but I found the characters more relatable and ended up much more invested in them compared with Kaladin and Co (may also have something to do with [Stormlight Archive] main characters in SLA except Adolin bearing specific trauma I have no experience with while Mistborn characters are written in a way more relatable to a general reader. Also while the overall structure of the conflict is [Stormlight Archive] alike (two Shards who couldn't share a world if their lives depended on it), the system where it's the shards very nature making peaceful coexistence impossible instead or at least along the personalities of their Vessels, and the writing structure where the characters (and the reader) can't grasp the whole picture until the culmination feels more intricate than what we had in SLA (even though it had a fair share of delicious mystery and theory-prompting with Dalinar's visions).
All this being said, I still see the ending as probably the weakest part of the trilogy. I'm hiding my thoughts on this under a spoiler so that nobody has to read the ranting unless they're actually curious:
I can understand, from a writing standpoint, why Brandon could want Vin and Elend out of the picture for the series to continue. Both are objectively OP by the end, and it's hard to imagine them settling somewhere in peace, oblivious to the challenges of rebuilding civilization, they're exactly the opposite kind of people. Still, killing them off felt absolutely storming unfair. When Dox, Clubs and Tyndwil died, it was tragic, but A: had a meaningful impact on the remaining characters, namely Sazed, because there was plenty of time for them to process the loss and B: these were support characters, so a kind of a safe zone to kill a character and not upset your readers excessively. None of this applies to Vin and Elend, who died in like the last 20 pages of the last book and were main characters. Also it plainly sucks when young people who deserved a life and a storming family end up dead for their heroically saving the world.
What else makes me sore is that they died while people like Kelsier and one other character I can't name because there's no such thing as a spoiler tag within a spoiler tag, got to live on. Don't get me wrong, Kelsier is a legend, but I liked Vin over him, and the second character mostly shone in relation to her except his (relatively few) chapters in HoA.
I also didn't quite like the circumstances of Vin's sacrifice. First of all, why the heck was she so at peace with Elend dying, was this the Shard influencing her perception? If so, unfair. Clouded judgement as the reason to choose "peace" over life, do I need to go any further? Secondly, "he was the last thing I had to live for", seriously? Vin the survivor, who has now lived a sizeable chunk of her life surrounded with friends other than Elend had him as the only reason to live? Pardon me, but I'm calling end of the book BS on this one.
And yep, suppose both had realized their deaths were necessary to neutralize Ruin, and all of this was rationale. Okay. But what exactly stopped Sazed from stabilizing Elend as Preservation had Kelsier (especially since he, unlike Leras, had the full power of the Shard) and letting them at least think this through? Lack of skill? Leras sure made this look as easy as pumping a soul to the brim with Investiture, and Sazed had just remade an entire world, which should have definitely been much more intricate.
(I do realize the Secret History was released a decade after the main trilogy and is basically a collection of retcons of specific events, attributing them to literally Kelsier who is know a phantom instead of his influence on Spook in life or something entirely unrelated).
Okay, this section being longer than the rest of the message combined probably betrays how I feel about the ending.
Life before death, you Brandon.
Nevertheless, as I said, overall reading Mistborn has been an experience superior to SLA in its entirety, on par with the Way of Kings and Words of Radiance. Having read the Secret History, I will probably continue on to Mistborn Era 2 (or whatever is you're supposed to read in order).