I agree with this thread, though I do have some suggestions.
For one, I don't think we should add explicit ratings in their tiering. Saitama and Garou may be able to grow/copy up to 3-A, however, they have never shown that degree of power, thus I do not think we can add a "tier" for that. I don't really think it works like that.
I think the better idea would just be to say something more vague like "potentially far higher with Accelerated Development" and define all the specific details of Saitama and Garou's limitless growth.
As for Garou's copying. It can just remain as "Varies." The same as something like Ditto. Though it can also have an explanation that explains that he doesn't have a finite cap on what we can copy.
Also, people seem to be misunderstanding what "Limitless" and "Infinite" potential mean. It doesn't mean that Saitama/Garou can grow to Infinity, and I don't think anyone is arguing that. It just means that their potential growth doesn't have a finite cap or limit, and can extend upwards forever. Similar to an exponential function that's graphed, the only real limitation on how high they can go is the amount of time they have to grow.
Another misconception is that Garou was unable to copy Saitama because he was too powerful. This is not the case. What actually happened was that Garou's copying was unable to keep pace with Saitama's evolution. He was falling behind, as in, in time, rather than in strength. Saitama grew so fast that he could no longer copy him fast enough to keep up.
We can actually see on the graph that Garou's growth is pretty much the same as Saitama's but with a slight delay. It has the same curve and rate of growth, but is slightly off-put on the x-axis, which is supporting evidence for what Garou was saying there. I also highly, highly doubt that Garou, who has knowledge of all energy, was prepared to go into a limitless copying evolution battle without knowing he had some arbitrary copying limitation that'd leave him capped out at some certain level. That wouldn't make much sense.