There's also a few intermediary 'solar system replacement' mods, if you want to go super-ham you can go with RO [Realism Overhaul] which essentially just plops the real solar system [to scale, as far as I know] into the game, but it's extremely challenging and basically requires a whole constellation of modded parts because with the scales involved, the stock KSP parts really can't get it done, they're geared toward the stock system and nothing really exists at that scale in stock KSP. The one I've been playing is KSRSS, which is basically the real solar system but at stock KSP scale, so you get the real planets of our solar system but it isn't punishingly difficult. Although I will say, Mercury is still a big bitch to land anything on, because the relative positions of the planets are the same as they are in real life, it's just the mass/scale that is different. There are also configuration options that you can tailor to how you play, but you have to have a general idea of what you like before really monkeying around with that stuff. Personally, I tend to play without the comms stuff [needing to have relays and such to actually control/communicate with your spacecraft] most of the time because I find launching endless comms satellites and adjusting their orbits to be tedious, but depending on your flavor of space autism that shit might be kino to you, basically setting up infrastructure for your future missions. Maybe I'm a filthy casual for playing with that requirement off, but I got really tired of trying to land my probe or perform a time-sensitive maneuver and I couldn't because I was slightly out of line-of-sight or a moon was in the way between the relay and my lander or whatever.
When it comes to modding, I'm the sort of motherfucker that starts modding a game and very quickly thereafter, I realize that I'm actually spending more time fucking around with mods [and troubleshooting why the fuck they keep crashing my goddamn game] than I am actually playing the fucking game, and likewise I would say that roughly 60% of my playtime in KSP has been spent in the fucking VAB or screwing around with spaceplanes in atmosphere for no particular reason, as most of the time IME spaceplanes don't really accomplish any scientific goals that wouldn't be better served with a regular launch vehicle. Main plus to spaceplanes is, if you do it right, you can have an almost 100% reusable launch vehicle, especially once you unlock the late tech tree stuff and can pull of a single-stage-to-orbit spacecraft, so you save a shit-ton of funds when all you really 'expend' during a mission is your payload. I will say though, KSP is actually one of the games I've played that takes very well to modding, I don't tend to have to troubleshoot it nearly as often and CKAN makes it so piss easy, almost too easy because I'm constantly downloading more shit that I probably don't really need.
I would say though, KSP is easily one of the few games where I never walk away feeling like my playtime was a waste or that I wasn't feeling it. And it does unironically teach you shit about spaceflight and orbital mechanics. It's really a fucking travesty that KSP isn't part of the public school curriculum at this point in terms of physics class or science class, because it genuinely does make learning these concepts fun and rewarding. Plus, with certain mod packs, you can use actual, genuine parts that have been flown on real spacecraft, so you can also learn about the history of spaceflight in the process. Plus you can sit in the VAB and build a replica of all of the cool shit NASA pitched and never built/flew over the years. One of the most fun playthroughs I've had was a run where I was seeing how far ahead of the real U.S space program I could get, starting in 1960, but of course there's really no comparison since you don't have congress in the way, nor "Whitey On The Moon" types shitting on the space program. Although actually I think you can mod that in if you really want to - correct me if I'm wrong but I'm like 90% certain there is a mod for political will/favor and public image/PR, and some of that actually does factor in to the career mode.
Overall KSP is fucking incredible and it's a shame that KSP2 was such a fucking shitshow that did not build on that foundation very well. As much as I love KSP, it's a game that started development in like 2011, and in some ways it definitely is showing its age and it becomes clear in some cases that portions of the physics engine are hacked together in the way that they work, KSP2 was an opportunity to fix some of those foundational issues and build upon them but they fumbled it so fucking historically hard.
