For the record, you can abuse the shit out of Rogue's bonus action sneak to break the Matriach's AI and just sneak attack it to death solo. For every difficult fight there is an equally effective cheese. You can also kill the two phase spiders and all of the hatchlings before engaging the Matriarch.
Additionally, if you're getting stomped, just come back later with better gear, better tactics, and elixirs of poison resistance. The spider queen is not a hard fight and it sounds like you're bashing your head against a wall by attempting the same thing over and over again. There are a lot of tools at your disposal to turn this fight from a grueling battle to a brisk tussle, you just seem to be ignoring them.
I agree, whole-fucking-heartedly, that her reward is counter in value to the difficulty of the fight. The robe has very little monetary value and has too niche an application to consider killing her for anything other than the exp. Melf's First Staff would be a much better reward and remains thematic.
Git gud maybe and learn how to kill the spider.
