You CAN do everything but raids on solo, depending on your skill, and you can even solo some raids if you're cracked. This game is meant to be a social one though. They want you to make friends and squad up. You will need to be pretty good to complete "everything but raids" solo. All that to say, if you're going in with the mindset of never making friends or squadding up, you are YOURSELF making the game more difficult than it needs to be and handicapping yourself. If you're cool with that, great, bombs away, just don't come back upset that the solo experience is difficult. That won't be the games fault.