[2020] Thinking of starting a modded playthrough.

I'd say K2 is closest to vanilla, just changing a bunch of things, adding a few science packs making the game longer, and adding some IMO unnecessary bloat like loaders/warehouses/inserters able to drop on close side. Overall I'd say it's easier than vanilla, as you have tools to solve basically anything. Multiple things I consider completely broken, like pollution filtering to get rid of biter attacks for minimal price, extreme lengths of underground belts and pipes to the point it's really hard to see which one connects to which. The conversion of resources to matter is pretty cool if you choose to use that. I finished K2 in about 60 hours.

Industrial revolution (I haven't played IR2 yet so I just assume some core things carry over) is interesting because it adds so many intermediate recipes that hand crafting really is really painful, motivating you to automate more. Progressing through the stages is also quite fun so that's not a bad idea to start with either. I didn't get to the total end of IR1, but I spent about 75 hours and got fairly close to the end of the tech tree. If you're a purist like me, who loves routing belts and do a lot of automation of a lot of things, IR is awesome.

Space Exploration is seriously difficult, but EXTREMELY good. Vast majority of the recipes are very well balanced and interesting, the progression is done very well, and it generally makes things more difficult, rather than less. Getting to the rocket isn't much harder than in base game until you get to launch rockets, but after that it's progressively more difficult, with the end being absolutely brutal. However, it's also by far the most interesting mod that does much, much more than just add more recipes. Even the recipes it adds are generally more interesting, but there's multiple things that are completely fresh and unparalelled, my top 3 is figuring a way to do cargo rockets, figuring out and building automated spaceships, and the end game challenge of arcospheres. Worth noting that all three of those challenges are pretty much impossible without combinators. I'm by no means a combinator genius, all of my logic is very simple and straight forward, but I very much enjoyed the challenge. I'd leave space exploration for when you're really confident, want an intense experience, and want to commit multiple hundred hours to a playthrough.