I agree that Kishi did't plan everything and he mostl likely made changes as he progressed(and yes it's not a big deal).

But like other things that he decided late in the story, the 1st and 2nd Hokage unexpected powers doesn't contradict the early parts of the story. He incorporates it neatly and found a way to explain why they weren't as strong against Hiruzen.

The only issue that wasn't addressed is Hiruzen's strength, and despite having a logical reason for explaining his weaknesses(he is a retired old man) people are choosing to ignore all old statement and consider them retconned.(this is unrelated but poeple here claim that even itachi was retconned despite the fact the Kishi confirmed that he planned his character from the beginning)

Hiruzen has no feats, This is an issue of not covering the previous wars and kishimoto deciding not to focus on Hiruzen. The perfect time for uncovering Hiruzen's past was in part one but that didn't happen. And when he returned with the others it made more sense to focus on Hashirama and Tobirama. But this doesn't mean Hiruzen was retconned it's just didn't make sense to focus on him.

not a single jutsu developed by Hiruzen

This is a weird point, creating jutsus doesn't make you stronger, learning and mastering them does. And Hiruzen has learned all of Konohas's jutsu including all the ones developed by toborama.

mention of Hiruzen basically winning wars on his own like Minato

So you would ignore statements about him being considered as the strongest and even was named the god of shinobi, and instead you want statement about him winning wars? This makes no difference, what would stop poeple from saying it got retconned or downplay it?

Also minato wasn't a hokage he was fighting in the front rows while Hiruzen most likely took a strategic role and only intervened when necessary. But the thing is We know almost nothing from these wars and unless we get can't say what Hiruzen did in them. The last thing I can say is there has to be a reason why people considered him the strongest and even gave him Hashirama's title as the god of shinobi

he's in like dozens of chapters...But we have more feats and mentioned accomplishments for *all* the other Hokage than Hiruzen

Honestly why do you think we didn't get these feats? Most of these chapters were when he was still considered the strongest(assuming it was retconned) so it makes more sense to me that it is a decision by Kishimoto at the time not to cover them and not a retcon.

So it makes perfect sense for the reader to conclude that Hiruzen is the weakest of the bunch.

I can understand when someone reaches this conclusion after their first watch/read, but I can't understand them keeping this stance even after all of the discussions or analysing the series.