So... few things to unpack here.
First of all, the unlockable secret scenes are extras, they don't have any effect on the main story. So if you're not a fan of collectibles, just play through the game normally and that's it. And if you're the type of person who does enjoy collectibles, you can have fun finding them all.
Secondly, the game indeed doesn't tell you how to unlock everything as that would nullify the point. It wouldn't be an achievement if the game told you exactly how to get it. And if you still do need help with some of them, there is an official walkthrough available for patrons which covers pretty much every topic.
The weekend activities go up to five stages, and you will eventually be able to do multiple of them. If you want to unlock the secret scenes right now, you'll have to do multiple playthroughs, but this won't be necessary in the future when there are more weekend opportunities.
Lastly, imo things like collectibles and achievements are a great way to incentivize players to see more of the game, especially when you have many choices and paths. Otherwise a lot of the content will never be seen by many players. Still, it is all optional, if you don't enjoy trophy hunting and unlocking 100% of a game you really don't have to. You can still enjoy everything that the main story has to offer.