You're literally all over the spectrum there

• For Hydra, you only need around 200 ACC. More as you get to brutal/nightmare, but not considerably more. I rarely even pay attention to my accuracy for Hydra, since GH is normally more than enough, when coupled with substats
• Doom Tower needs a bit more - around 350
• G5 really depends on what team you're building. For someone like Serris, you probably want at least 600 ACC, because that's all she's doing. On the other hand, for someone like Lydia, around 350-400 should be fine.

I would want closer to 225-250 accuracy for Hard even in rotations where Suffering doesn't start out with the heads. 

Brutal, I'd want over 300 because of that, then 350+ for Nightmare, if not more.

It is for Lydia, want her as an Aura lead in go-second team. Debuffing the enemy as well as placing that fear if they try to control my champs. This is essentially to decide whether to go for ACC or Resist in the last mastery. 

She's 328 in artifacts, can probably glyph another 20 on her. Once I ascend her (waiting for CvC), I'll be able to put another 20-30 via Amulet. This is where things get funky - my RES banners are better than ACC banners, so wondering if I'll have enough accuracy to go around. 

As a start, I'd go with whatever banner gives her better substats, SPD being the priority. So, I assume for you that would be a RES banner. A tanky go-second team benefits more from resistence than accuracy anyway.

That being said, I would advise to just experiment. Try out all possible options. That way you'll be able to determine which particular build suits her best in a particular team. I tried out 6 or 7 different builds on mine. Good thing she likes dress-up, otherwise she would've left me by now.

Definitely plan on it. I want to start with a basic tanky debuffer/controller build. Focus on Speed, HP, DEF, ACC, and RES. Figure it's the most basic build for her, but I want to take full advantage of her kit's versatility. 

My suggestion - don't try to build her as a jack-of-all-trades. Her utility has gone downhill very significantly in arena from when she was originally introduced.

Her value in go-second teams is ... questionable, at best. The debuffs provide almost no value at all, so what you're mainly getting is a revive blocker and the buffs she provides. I would suggest not really bothering with accuracy at all, beyond what you get from substats and from GH. Versus go-first teams, she won't really need much accuracy at all to debuff them, and versus go-second teams, you're not going to win fights because of them.

Focus on making her just super, super tanky, because what she's mainly going to do is allow you to get a few turns in when you manage to kill someone, before they can get a revive off, and that's likely going to be what you'll need to actually win a fight. To be totally honest, I'm not even sure if I'd bother with that much resist, because if you're against go-second teams, they're not really going to be killing her either, assuming you are running some solid backup with her, like say, Mithrala.

Just get her fast, with high HP and DEF, and cycle her turns as often as possible. Keep buffing your team to cycle them, and to recycle your revive block cooldown.

On the flipside, if you're building her in go-first teams, then she needs no resist at all, and instead, you'll want her to have really, really high accuracy, so that you can debuff those go-second teams consistently. That means typically 500+ ACC, assuming you're not running someone with ACC up.

On Hell Hades Raid site he has got all the attributes of all the bosses, so for example if you want to know what accuracy or resistance the twins have on any floor any affinity you'll find it there

You do have to sign up for this feature but it's free and his whole site is very useful