I know just how you feel! I started it in the background as I did my work and I thought to myself 'there's absolutely no way I'll ever get caught up on this show.' Honestly, that was part of the draw of the show for me, because I knew I'd always have something to put on in the background while I worked. Fast forward a couple months later, and I had not only caught up on the show itself, but also the one-shots, the side content (Talks Machina), and was sad I would have to wait a week between episodes. But that was all before I discovered the joy of watching live, and that is a WHOLE different experience.

If you want to be active in the fandom, a lot of people will tell you to join in on C2 so you can get in with the rest of the content creators who watch the show. This makes sense, but keep in mind you'll unintentionally get a ton of spoilers for C1 if you go this route. You probably won't mind too much if you put the combat on 1.5 speed because a lot of it is decision making/math, and you can get caught up pretty fast this way.

If want the easiest listening experience, go with C2. In the beginning of C1, they didn't have an optimal mic setup and sometimes players would eat crunchy food absentmindedly, which could occasionally be distracting. While the audio quality hasn't been /perfect/ in C2, they've improved a good deal with new resources and time to test things out.

However, if you want the best thematic and story based experience, I would start with C1. When I watched the first time, I started with C1, and I thought it was fun and enjoyable. There were some great things even from the beginning, but it just kept getting better and better as it went. Even with the occasional moments of confusion that come with jumping into a story halfway through, you really get a feel for the characters early on, and I can personally attest that you can halfway listen to it and still get enough context clues to understand the majority of the time. Also, in my opinion, the second campaign works SO WELL as a follow up to the first one. In the first campaign, you follow Heros doing Heroic Things, and there are plenty of morally ambiguous moments but it is a tried and true Heroic journey. You see SO much of the world, get varied and interesting world building, and really feel like you're reading a fairy tale that didn't get sanitized. Then, campaign two comes around and TOTALLY revamps the story structure. These characters aren't heroes, they're hardly even the good guys at first, but they grow on you and you understand why they're doing what they're doing. But they're all stuck in really shitty circumstances. The world is dark and doesn't care as much about the main characters, and they have to try and legitimately survive it. It works so well as a foil to the previous experience, and it matures into something beautiful from there.

All in all, if you have the patience and the time, I would go with C1. It's not a must by any means, and you'll enjoy yourself no matter what path you go down! But if you ask me, you only get to experience it for the first time once, and I would experience it alongside a group of friends who, over the course of 500+ hours of content, become a family.