Your example of Cap holding his own against Thanos is the perfect example of the same problem you have with Wanda. One movie Cap struggles with Batroc the leaper and another he can stand with Thanos who wields 5/6 of the Infinity gems.
The Marvel movies are never going to have characters strengths and limitations consistent, just like their source material.
Well yes. It's definitely inconsistent. But I think in the Cap/Thanos example it's a bit excusable with narrative context. When Cap stands his ground against Thanos, he's in a situation where he's really become the second to last line of defense. If he can't stop Thanos' advance and he gets through Wanda, everything is lost. In that scenario it makes sense that you'd see Cap pushing the very limit of his strength that you wouldn't otherwise see. But the fact that he doesn't overpower Thanos and merely holds him off for a few seconds, inviting Thanos to actually try to incapacitate him is an indicator of his limitations.
When Wanda is on the verge of killing Thanos in the final battle its a channeling of all her vengeful rage that could've killed Thanos if she hadn't been interrupted. But Thanos was still able to resist all her power for a while so there's a limitation there, even if it rests Wanda pretty squarely among the three most powerful avengers.
Here the only limitation we get is that she had a pretty fairly matched fight with Agatha. But the only reason that could happen is Agatha was capable of absorbing and siphoning Wanda's power. Past that, Wanda's grief manifested so strongly that she distorted the reality of an entire town, possessed the minds of all its inhabitants, and created physical manifestations of Vision and their children that never really were.
I mean that's more or less Wanda pulling reality stone shenanigans without the benefit of an infinity stone. And she's only supposed to become stronger now that she's embraced the power of the Scarlet Witch. I can deal with a little power inconsistency to service plot and action. But this, to me, is too far. Anything that happens in the MCU going forward will be tinged with "if Wanda were here it'd be over in a heartbeat" and I'm sure they'll use the excuse of "Wanda can't be found" as she studies in isolation but eventually she'll have to figure back into things and she'll basically be a goddess when she does. Like at this point I don't see how anyone is even close to being the strongest Avenger.
It's like the reverse of X-men apocalypse where, aside from other issues with the film, Apocalypse is shown to be so absurdly powerful that it makes the X-Men's triumph over him feel contrived and convenient. You can't have Wanda face a compelling adversity now without either having a villain or threat that is ludicrously powerful or retcon the increase in the power she gained in this series.
Like I get what they were going for with the Hex in telling a grandiose tale of grief and loss and it worked in the bubble of this series. But the implications it has going forward are rough. I kinda wish there had been some other force driving the Hex. Even it had mean Agatha was behind all of it. Or Agatha's presence amplified Wanda's capabilities to a degree that made the events of the show possible. Instead we watch 9 episodes of Wanda's power amplified grief that culminates in her going forward as basically a magical goddess with immense power. Immense power that I think steps a bit too far out of what is compelling and enjoyable.