I just recently picked this game up and from what I can tell with the change notes that were given (was trying to get a sense of when the demo stopped) a lot of fairly drastic changes have occurred to make it more tolerable for solo-play. Just from what I experienced, however, (got a bit stonewalled by the 12-18 level catacombs due to Fire Slimes having unrelenting DoT) I think I can get an idea of why the steps were taken, and at least from my perspective of how other games deal with 1v+ encounters or genres, there's more steps that could be taken to further a vision where that can be balanced with team-play setups. Given the game's selling point is already a bit of an exotic cross-genre approach that's combining elements of action-RPGs like Zelda or Soulslike, and MMOs with enemy-aggro and detection mechanics, mission/quest setups, and intent for teams - I think it's acceptable to consider influences from a few other sources.

• Beat-em-Up games: Power in Ambiguity, and Target Slots - So one thing I notice about Atlyss is that most every enemy, other than the miniboss and boss, unambiguously commit to pursuing the player and unleashing an attack from their moveset when they're ready - some of it dependent on their distance from the player, but the ultimate outcome ends up approaching the same: either a stack of several enemies repeatedly hitting the player, or a stack of several enemies repeatedly getting stunned by the player at the same time due to a generous parry window. They snowball into a mass around their target and this ends up exacerbating some issues that a lot of the later-demo designs have (the models are so big the player can't tell what's going on!) So what good examples of this genre of game will do (streets of rage, final fight, double dragon, god hand, sleeping dogs...) is that each player will have a set of target slots that can be occupied by enemies - basically locations to the left and right (or all around the target in 3d games). Basically this conveys not only a framework to tell the enemy which player they're going after (hey good tie-in with potential MMO aggro mechanics?) but it also can coordinate what everyone else in those slots is or should be doing - if we wanted to make it so that parrying had a shorter window again, this could be used to help space out the attacks instead of having them occur in rapid succession. And what happens if enemies enter or exit that list of slots? If there's no room or if they've moved out of position? They orbit, posture, loiter, wait their turn to get back in there. There's a really good video that I found that puts into words how/why this works, great 15-minute watch that can get gears turning for implementing clean 1vX battle dynamics.
• Metroidvanias: Foe Variety - This honestly could just as well be covered with the above genre, but I say, heck why not this instead? It's more noticeable here because most entries of this genre aren't going to have nearly as many entities fighting you all at once, which then makes the composition of what's being fought more noticeable. Examples of this can be seen in games like Symphony of the Night or Bloodstained where specific room layouts will challenge the player based on the combination of two tactically diverse enemies - maybe some Skeleton Musketeers stationed behind an Armor Lord, forcing Alucard to time his jumps, approach from above and cross them up. In Atlyss where the player's deliberately given options for both melee and ranged combat, this kind of variety in enemy loadouts should be heavily encouraged! ESPECIALLY if team-play is a priority, because this now would mean that, hey, if perhaps an encounter that used to be two of those death guys, was instead just one of them, and then some kind of golem turret dude - then a duo of players, maybe one a Fighter and the other a Bandit, could split targets between the two.
• General: Input Latency - Maybe partially due to how the combat behaves at the moment, but the matter of the fact is that it's currently  highly-reaction-based. Parrying is such an important aspect to master that being ready to use it on a moment's notice is probably the most important part of winning any fight. As a result my first and utmost focus in this situation, to make the fight take shorter, is to have the attack button held down, and this has a couple of issues concerning some of the cooler toys players get:
  
  • Charge Attacks are inherently laggy, get interrupted by blocking, and sometimes their attack animation's outcome (as I found out with katar charge attacks...) create a bigger opening on you than if you hadn't just continued attacking normally. It almost NEEDS a parry-stun lead-in to work. The fact it can be held indefinitely (?) also means that maximizing its DPS versus just standard slashes is really rough to time and that sour-charges represent a massive loss on that front, as well as for your stamina. I'd just make this work like how DS3 onwards did heavy attacks - you can charge them, and they go off automatically once ready.
  • Skills are deliberately greyed out while using normal attacks and their use cases usually aren't consistent from skill to skill - felt this the most with the Fighter Loadout, with Stomp being a charge that would automatically channel once charged, but required you to be on the ground to even initiate, then Lethal Slice being a charge that isn't auto-activated on full-charge and can be used in-air... So in order to use any skill I would need to deliberately stop using my attacks (which often were passively contributing to building up stun on my enemies) and take massive risks of being hit for what might be a bigger burst of damage across a larger spread of oncoming enemies. Frankly this should be taking pages from MMOs that have auto-attacking - your fancy skills interrupt your basic attacks, not the other way around.

Anyways hope any of that rambling made sense and/or was tactful at all. I think aside from such issues everything else about the game I've seen seems really neat.

I agree with basically everything here. I found the 12-18 dungeon an extremely frustrating experience due to the difficulty spike. I imagine the full game will be much longer with a more gradual difficulty curve, but it should not be demanding such an extreme level of mastery from the get-go. Frankly, I think the first real boss (the haunter-looking guy) isn't reasonable for a first-time solo player to deal with, since he has multiple very fast, extremely wide-reaching attacks that hit multiple times and can potentially one-shot a player who hasn't invested significantly into vitality- which I don't think most players will, since not much in the game up until this point will hurt them that much. The first boss should be a check to see if the player understands the basics of the game, not a check to see if the player went and bought a dozen health potions to facetank the boss.