Yes, you can. There are a lot of side activities, collectibles, and challenges that you can still go back and complete even after finishing the main story. Things like hunting all the legendary animals, finding all the cigarette cards, and doing all the stranger missions.
Sure. Completing the story doesn't prevent you from achieving 100% completion in RDR2. You'll still be able to explore the vast open world, engage in activities such as fishing, horse taming, and interacting with the various non - player characters in the game. There are also numerous trophies or achievements related to things like perfecting your shooting skills or completing all the mini - games that you can work towards.
Definitely. After you've completed the main story in RDR2, the game world remains open for you to fully explore and complete all the tasks required for 100% completion. There are a ton of things left to do. For example, you might not have fully explored all the regions of the map during the main story. Now you can take your time to discover all the hidden areas, complete all the bounty hunter missions, and upgrade all your equipment to its maximum. There are also a bunch of side quests and optional activities that contribute to that 100% mark. So, don't worry, the game has plenty to offer even after the story is done.