Why though. What makes him trust Sidious. Just that the Jedi won’t let him into the vaults of the archives and secrets about the Sith?

Sidious could be lying about the secret to immortality and to preserve life.

Sith do lie for their own ends.

The Jedi also lie if you follow the life of Obi-Wan Ben Kenobi. His case is different, had he told Luke the truth. Would Luke have helped him on a mission to rescue the princess? Joined the rebellion?

The part that makes no sense is not telling him Leia was his sister, that’s on Lucas for making it up in the Throne room duel to solve an issue. Why lure Luke off Tatooine with Leia’s message because Luke is infatuated with her. Clearly a case of a boyhood crush. One that makes him accept the call to adventure, only after Owen and Beru are slain.