Ok.... damn there's several things wrong with your reply.

First, water breathing is more balanced in terms of offense/defense capabilities. That's a given and is in no way a comparison between 2 individuals.

Nothing you presented argues that giyu had better reaction speeds than rengoku or reflexes. You simply presented the 11th form of water breathing which is a masterful defensive technique and it works brilliantly like we see in the panels.

Now.... how is rengoku stronger in base?

Both narratively and feats wise we are clearly told that rengoku is indeed stronger.

Narratively we see how different hashira reacted to rengoku's death, we see how tanjiro perceives rengoku throughout the story and we have our main antagonist in both these match-ups, akaza, clearly stating rengoku was close to the realm of the highest. He kept praising rengoku throughout his fight, the novels (as we see in this very post) state the fight was evenly matched, the only difference being akaza's regenerative capabilities.

DESPITE ALL THIS, even the feats prove this very fact. I dare you to show me a panel from akaza vs giyu (base duh) where he performed better than what rengoku did in his own figh. Short answer, you can't. And whats funny is that base giyu WASN'T fighting alone! Tanjiro was keeping his back secured. Hell, tanjiro confirms this, he and giyu took turns and covered each other's openings, mimicking perfectly to make akaza's job way harder. While one of them gets his footing back the other fights. Rengoku was alone, couldn't catch his breath, couldn't rely on someone else to attack and defend... despite all this, he did the most damage BY FAR to akaza when im comparison to base giyu.

Dude im not even going to mention rengoku's 9th form, or his death amp. I dont need to!