Styanax is a beast in Circuit since his Final Stand spears scale off the amount of decrees you have. He's easily one of the best Circuit frames, standing alongside Nova, Vauban, Gyre and Protea for hard-scaling Circuit nukers.
To be fair, Styanax is a beast everywhere due to the raw power of Final Stand (especially augmented with Overguard) and Tharros Strike. If you're not in Duviri, the projectile ability scaling can be tapped with Arcane Arachne.
I was on Switch at the time, and even with the godawful load times it was my favourite content. Whenever I had relics to crack I'd drag the clan with me into Void Storms, or even just solo Corpus Void Storms. The thing that killed Railjack for me was the introduction of SP fissures, since the value proposition of doing a soft-endurance endless fissure is way ahead of anything else in the game.
I'd still recommend Void Storms in a heartbeat to new players though. They're hugely valuable everything farms with credits, endo, and preradded relics.
As for Railjack itself, I really like the quality of life of universal affinity and shared loot, as well as the collaborative nature of the objectives. It's cool playing with my friends and operating as a team, which is basically unique in Warframe in its current state.
I like your idea. Instead of Overguard I think your idea of exceptional DR plus party-wide status immunity would put her on par with Dante's utility.
I misremembered, Steve read out someone who commented that during the preview devstream: https://www.youtube.com/watch?v=T6Vy7OkcXc8
Madurai helps with that too, since Power Transfer gives you faster casting, and you can naturally get Sling Strength by rapidly void dashing to the first pack of enemies.
Gyre herself is the "nobody else gets to play" frame. I run 155% duration so I just need to get a handful of kills at the start and the buffs are self-sustaining. Enemies only need to be tagged by Rotorswell or Coil Horizon to get the duration extension, so after a point you just have to keep up and keep throwing Coil Horizon.
Yeah, while it's a powerful nuke, it works best with her 2. It also has redundancy with her 4 in that both are "deal electric damage". I've tried replacing her 2, but the utility of an absolutely massive grouping ability is too hard to pass up.
If you don't like shield gating and want a more passive build for general play, eclipse+primed redirection+arcane aegis+adaptation passively tanks for a while, although I haven't tried to see how far it scales. Her damage output and CC are good enough that she doesn't actually take that much damage to begin with.
If you want to avoid the squishiness, the new holy trinity of passive defensive tools is Eclipse+Primed Redirection+Arcane Aegis, but Health Conversion+Arcane Blessing+Fractured Blast/Lycath's Hunt is still good. The Eclipse build can tank really well for a few hundred levels, and it's my daily driver Gyre build. Anything more and Pillage sadly does just kind of win for survivability, and is what I use for Circuit.
Primed Redirection on Pillage is super comfy now, since you're basically always at your 2.5s shieldgate.
In the immortal words of Pablo, "Skirt goes up, enemies go down"
Limbo has been through a few iterations, how about his current state and how the game has changed to give him tools to mitigate his pain points? For example, Zariman was kind of the Limbocalypse due to Overguard, but he has Fortifier and Magnetic to deal with that mechanic now. I also didn't join until 2015/didn't get seriously into Warframe until 2021, so I personally just can't speak for launch Limbo.