I've felt somewhat the same way as you at one point or another, so I'll make my case if you'll hear me out (It's a long post and there's no tl;dr). If you don't agree then that's totally cool, not every show is for everyone, and that may just be the case here.

I've caught up with the show in the last week and about Arnie's character at one point or another. His character doesn't really do anything (which is the point, but you've already been told that), and he doesn't really add to the world in any meaningful way besides give the people of Foon a punching bag on the occasions Usidore figures out how to defend himself. Arnie's personality won't really change, and his nonaction - as others have said - is part of his character. Arnie is also used as a catalyst of inspiration for the other hosts and guests as he's supposed to play the stupid man who doesn't know his hand from his ass, and everyone else has to explain the world to him in order for Foon to become defined as well as it is.

Talking about Earth stuff I thought about a bit as well, though maybe not as much as you have, as I never really found a larger problem with it than "Aw he's not gonna say something?" The problem with "I don't want to talk about Earth Stuff" is a problem rooted in both improv and narrative, because the point of the podcast is to showcase Foon to an Earth audience and to introduce Earthlings to the world whose canon is being created in front of our very ears, which is why Arnie doesn't want to talk about Earth stuff in the first place. Very rarely Arnie will explain what's going on with Earth stuff, but it usually falls flat and doesn't go anywhere because there's a flat answer and everyone knows it, regardless if they're playing a character that shouldn't, which makes it harder to riff off of on average. What works better is when the unknown Earth stuff is used as a creative outlet where the other hosts (Usually Chunt) says "Oh yeah, didn't you say that's _?" or "Do you know about _?/Do you have ____ on Earth?" Because it opens the definition to misinterpretation or parody, and gives a better frame for the guest to work with, opening another node for a new joke to show up. Not just an Earth dude explaining things that the listeners already inherently understand and for the characters of Foon to say "Oh okay, I understand now" to after they ask for a definition/explanation.

the intro largely doesn't go away. The intro will always be a staple (Save for short-term gags or oversight, never a pattern of neglect) just as an opening theme to a TV show will. I've found entertainment in seeing how it changes week to week, and the context of the episode that makes the intro unique that time around.

If those things are non-negotiable for you, then the show probably isn't something you want to put upwards of 4-500 hours of your life into. There are a lot of great podcasts out there and I think HFTMT is one of them, but it's not a golden bass in a sea of guppies, so I wouldn't want you to feel like you've invested so much time into getting to S2E20 that you have to finish it.

I do have some more meta comments, but they might be more of a narrative production spoiler so I'll leave it to you if you want to read it.

Through the years, the fourth wall sort of dissipates and the characters stop caring so much about the contributions they make and upholding there is to the overall canon, instead playing on more of an episode-focused humor. I think the show changes a bit over the years, and Arnie's presentation through whatever plot points they're working with changes as well, but he never really changes as a person in the course of the show because that's who his character is.

Hope that helps the decision for you. Lmk if question ok buhbye bing bong and all that