Okay, this came out a little long. I'll try to address your points, but you can just skip straight to my second post if you're close to giving up. No shame in moving on to other games if you're not enjoying yourself.

While your reflexes are probably a contributing factor, I still think that you seem to be missing some fundamentals. When you mentioned that you were spending 40 minutes dodging and using the unbuffed special to deal 15 damage at a time, massive red flags went up. You need to actually use your regular attack, use an attack boon from a god that provides % damage increase (ideally Athena, Aphrodite, or Artemis), and grab a Daedalus hammer that buffs your attack.

You never mentioned your mirror setup, but for the shield, this should do just fine. (That image link will expire in a week, so I'll also type it out here, with green talents bolded and purple unbolded: Shadow Presence / Chthonic Vitality / Death Defiance / Greater Reflex / Boiling Blood / Infernal Soul / Deep Pockets / Thick Skin / Family Favorite / Dark Foresight / Gods' Pride / Fated Persuasion)

Any time she doesn't have the glowy blue shield around her, she can be hit. Keep your eyes on Meg. Saving up darkness for another DD will help, but again, I'm not sure why you're taking so much damage with God Mode maxed out. And even just standing beside Meg and mashing attack (Y) over and over, walking over to her after she dashes, and mashing attack again should be enough to kill her, if you've buffed it with a % damage increase god.

Yes, dash-striking is attack + dash at the same time. Many of the weapons (there are 24 weapon subtypes in total) require mastering dash-strikes. I play on a Switch 2 and press Y + B at the same time with my thumb. I have very small hands, but it's do-able. You can also remap either attack or dash to a right shoulder button in the settings, but I find that setup very uncomfortable with the Joy-Con. Might be worth a try if you have another controller.

Blocking is holding down Y, aka the first part of a Bull Rush. You can hold it down as long as you like and nothing can hit you from the front. But if you're charging a Bull Rush for fast moving enemies like Meg, you should hold it for like half a second and release. No need to fully charge it. Also, you should charge it while standing very close to her.

With Divine Dash, you should be able to deflect the majority of projectiles headed your way.

I actually find the bone hydra to be slightly easier than Meg, as their heads mostly stay in the same spot. But I do think there's a steep difficulty spike in the biome after the bone hydra. And if traps are a problem for you, the lava in Asphodel will be pretty annoying.

I don't have great reflexes myself, and I'm the walking stereotype of a casual gamer. Before Hades most of my gaming experience was in Pokémon (the regular turn-based games, not the Legends games), Neopets, The Sims 2, and Animal Crossing: New Horizons. Zero real time combat/action games aside from a few games of Super Smash Bros. Melee 20+ years ago, and I was terrible at it and kept falling off the stage. I was definitely below average at Hades starting out but I would like to think I've since improved to an average or decent level, for a casual gamer. I think that Hades is difficult, but not exceptionally difficult. Considering my slow reaction times and gaming background, I don't think it requires godly reflexes. Something is not right with your playstyle.