Garp does not practice busting mountains for High Tiers. Get real.
And again Luffy just needed better Haki vs. Kaido. He was already close to Kaido, just like he was very close to Katakuri pre-FS. Him getting "owned" before is just Oda creating fake hype. Characters don't go from High Tier to matching Kaido 1 v. 1 because of 1 (or 2) powerup.
Why? We don't know how Prime Garp kept his shape and what training methods he used. Also, we don't know about Garp's intention
how to beat him. If he busted mountains to strengthen his punching strength to
beat Chinjao at his own game - therefore, a very crushing defeat - it does not necessarily mean that Chinjao matched him in general prior to their last battle.
Luffy had a whole mini arc dedicated to strength + Haki training. Do you think Oda randomly drew his seacuffs training for no reason? It's a benefit for him. Base Luffy partially dodged Kaido's TB but it still hit his head. Do you remember their first battle? This attack knocked him out in his strongest mode while base Luffy struggled this off.
You're lowballing Luffy's PU but those 2 (3 if we count the seacuffs training) PUs massively pushed him at the end. Which character can just randomly learn very advanced levels of Haki in a few weeks or even better, within a few minutes?