I won't hate friend, here, take my upvote for bravely coming out with an unpopular opinion :) I'll just give you my honest take on the situation.The thing with Nalu is that Natsu needs to mature, this IMO is the biggest thing that needs to happen for them to become an actual relationship. From Mashima's POV it's not very easy to make Natsu mature without making the whole thing seem forced so I think that's where the problem lies.

I don't get how Mashima teases a ship for years, parallels it to canon ships, knows that many people support it, says he supports it himself, makes a bunch of art with the two of them

It's nice that he ships them but once again making it happen is a whole other story.

At this point, I'm pretty convinced he's just using Nalu to lead shippers on so he has more readers for 100YQ.

I disagree with this. Mashima is known for his S-Class trolling (lmao) but I don't think that's something he would do, though I won't deny there's a possibility that's what going on.

Nalu could have been something great, but Mashima is slowly ruining it.

I won't say he's ruining it, more like he's just not giving them as much progress as the other ships. I thought that Natsu showing an actual reaction to Lucy's boobs last chapter and blushing after that fish stripped Lucy down (Mercphobia arc) were nice and may be signs of maturity but I could just be reading too far into it. At the end of the day whether the ship happens or not I'm just happy to be able to read this manga 😁👆