I agree with OP.
The issue with using Ryusuke's statements is that Ryusuke is
unreliable and his statement about pseudo-R>F contradicts more credible information we have in Honkai. Ryusuke
obviously betrayed Welt and was being deceitful with pretty much everyone he interacted with. Even after "explaining" his plans, he even
tried to
trick Welt
into thinking he instantly cut off all of Welt's limbs when
in reality, he was just moving stuff around in the higher dimensional space to make it look like that (
Note that Welt was still able to curl his fist after it was "severed", which shows that it wasn't really cut off). It's far more likely that Ryusuke was just bluffing.
Additionally, contrary to your claim that Welt's singularity rebuild explains why Welt was able to defeat Ryusuke despite supposedly being uncountably infinitely weaker than him, that doesn't solve all the inconsistencies. Namely,
Welt was able to block one of Ryusuke's attacks not with his gravity powers, but with an imaginary barrier. So what's going on here? The imaginary barrier
previously got bypassed entirely by Ryusuke's attacks, which seemed to support the idea that Ryusuke had pseudo R>F over him. But then, Welt
reframed his perspective and looked at the bigger picture past the illusions caused by the higher dimensions. He not only discovers that he was in one piece the whole time, but is also able to block one of Ryusuke's attacks with his imaginary barrier. The fact that all Welt had to do to be able to do this was reframe his perspective/recalibrate his senses rather than power himself up physically shows that Ryusuke didn't actually have uncountably infinitely stronger AP than Welt, he was just using spacetime hax to go around Welt's defenses (which Welt was eventually able to properly defend against).
On top of all of that, Ryusuke's statement simply contradicts the credible information we do have about higher dimensions in Honkai Impact.
Here, it straight up says that only real space (which is explicitly said to only be 4-dimensional even when counting the dimension of time!) is capable of producing any sort of real physical meaning, and all higher dimensional spaces are incapable of producing meaning, physically interacting with realspace objects, or having the attributes that characterize any sort of physicality (raw text can also be seen
here). It's literally impossible for Honkai Impact to have any significant higher dimensions because all of the higher dimensional spaces explicitly lack the characteristics necessary for Low 1-C to Low 1-A to begin with.
So overall, we should in fact downgrade Honkai to tier 2 at best. Not only is the only statement supporting Tier 1 a non-credible bluff, but it also contradicts the actually credible information we do have that confirms that significant higher dimensions simply do not exist in the verse.