Aight so I'm going to proceed under the assumption that the Hollowfied Bankai defeating Poisoned Ritter point has been resolved, as damage only brought mention to the Yama point being left unresolved. If you'd like I can drop a scan of every instance that adding Hollow powers to a Shinigami made them stronger to back up my point, but I feel that's excessive and we can just move on.
Yhwach says the following, "
It's not that we couldn't steal your Bankai. But who else can control your immense power besides me? That is why I ordered Royd to stand down until I arrived."
First, Yhwach implies that Yama's Bankai couldn't be stolen because no one could handle the power of his Bankai. He's not saying "no one is comparable to your base/Shikai form, hence they can't steal your Bankai", he is flat out saying that because no one compares to Yama's Bankai, no one can steal it.
Second, the immense power in question is directly referring to Yama's Bankai. Yhwach in base is capable of control Yama's Bankai. But can't base Yama control his Bankai!? No not at all, the Zanpakuto work by sealing one's power within a blade, meaning Yama can only handle his Bankai while in Bankai.
To give some examples of this,
Kenny can only handle his Bankai for a short time while in Bankai, and
Ichigo's Bankai started cracking his bones while in Bankai. The point is being able to perform Bankai is not the same as being able to control Bankai. Because Kenpachi and Ichigo weren't durable enough they couldn't control their own Bankai, yet Yhwach can control Yama's Bankai perfectly fine.
To further support Yhwach scaling to Bankai Yama,
he casually cuts Yama in half. When you couple this fact that Yhwach is hyper casually above Shikai Yama with his "I can control your Bankai's immense power" statement, it becomes rather clear that at the very least, Yhwach is comparable to Bankai Yama.
To bring up their original fight 1000 years ago, Yamamoto's Bankai failed to kill Yhwach. But Yama's Bankai was weaker back then! Lucky for us the statements made about Yama's Bankai would be applicable to his past Bankai, as that is the only thing characters like Unohana have to go off of. Yama himself says and proves that his Bankai has evolved since back then.
Now let's look at things from a lore and author intent perspective.
It isn't until after Yama dies that it is stated that the Royal Guard will show up. This would mean that Squad Zero does not see a reason to arrive on battle unless the threat is enough where even Yama cannot handle it. Implying that narratively Squad Zero are meant for threats greater than Yamamoto.
To invoke Occam's Razor, the simplest interpretation of "I can control your immense power" would be that "I" in this case scales to the "immense power".
The
Hollowfied Bankai vs Poisoned Ritter support the notion that the captains needed power beyond a Bankai to win, as they required to absorb Hollow power while simultaneously weakening the Ritter's spiritual powers. Which would then back up the fact that the Ritter who stole the Bankai are relative to said Bankai, because they were only ever beaten when nerfed and facing amped Bankai.