Interesting that you only bring up the points where Luffy lands attacks, but not points where he is on the retreat. But if you really need me to spell it out:
1)
Luffy clearly felt the need to defend against Doflamingo's kick with a culmination of Haki and Elasticity to make Doflamingo's Haki infused kick bounce off.
- What other point would there be for Luffy to use a defense mechanism like this if Doflamingo's attack wasn't going to harm him in the first place? Doflamingo immediately got back up and resumed the fight for an extended period and had the upper hand for parts of what we were able to see.
2) The battle went on for 20 minutes + however long it took for the following scenes to play out (the citizens reacting to the cage enclosure/losing hope, Riku's speech, and some panels of Doflamingo and Luffy fighting). Doflamingo can not be far weaker than Luffy if he's pressuring him for this long of a period.
3) Luffy continuously dodging Doflamingo's waves of threads is proof enough that those attacks are threatening enough to harm him, otherwise, why would he dodge instead of bursting through them like he did later against Doflamingo's shield just before Leo Bazooka?
1) Luffy landing some blows that did damage, then Doflamingo causing Luffy to retreat and evade nearly all of his attacks? The only one he tanked was a kick (which his Haki and Elasticity resisted against). If Doflamingo was so much weaker than Luffy, the latter would not need to continuously dodge all of his attacks in desperation while trying to look for an opening over the course of 20 minutes.
2) We already have to discuss Haki scaling anyways since there are points supporting the idea that Gear 2nd + Haki Luffy could get partial scaling to Doflamingo's Durability for inflicting damage to him (albeit only via dura negation on a healthy Doffy, and drawing blood on a clearly lethargic and severely wounded Doffy with Haki infused blows)
I still want to discuss the matter of 6-C~6-B Doflamingo but that's a whole topic in itself that I believe you are already heavily against anyways.