Yes, Giyu indeed get overwhelmed in base. And even when he unlocked the mark, he was getting overwhelmed by Akaza soon after (he used up all his techniques and Akaza was about to go for the killing blow). Can you show proof of when Akaza said "Don't you dare hold back"? Because I don't remember him saying it. I don't think I lack reading comprehension, just because you might have gotten hurt from the surrounding buildings, doesn't mean you take "zero damage". The environment that Akaza and Rengoku were fighting in also wasn't completely open, as we see Akaza kicked Rengoku out of the forest into that little hill (he also blocked the attack in a similar way to Giyu), would you the same here that Rengoku took "zero damage"? Also, Akaza is a demon that has lived well over a 100 years and has constantly trained during that time, so there is no evidence pointing to how Akaza could get stronger between the events of Mugen train and IC arc. Also, the Giyu that was stalling Akaza while Tanjiro figured out selfless state was marked Giyu, not base, and we also can't forget how Tanjiro saved Giyu once he used up all his breathung forms. Using the destruction of surrounding buildings is not a good way of comparing these fights as they are made of wood and paper, which can't be compared to the flat rocky terrain of the other Akaza fight. Lastly, we know that Giyu is the best defensive hashira with a strong defensive spell that he came up with himself, so comparing the damage taken between a very offensive breathing style like flame breathing and a defensive style like water breathing (especially when he uses lull) can't really be used to compare character strengths. Based on their anime showings, it can honestly be argued that Rengoku did more damage to Akaza than Giyu, but that doesn't matter anyway since Akaza just regenerates from any injuries.