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When Maul was Palpatine's apprentice- did he know what his acutual plans were (I.e. to command both the Separatists and the Republic and start a massive galactic war to create the Empire) or was only Dooku privy to that info??

Doubt it.

He just used Maul like a sledgehammer to carry out his bidding. No reason to show him the full picture.

Maul definitely knew something, as he wasn't surprised to learn that the Clone Wars had begun...

https://i.imgur.com/A5PlYwU.jpg
https://i.imgur.com/swgPMF7.jpg
https://i.imgur.com/hW5nbXM.jpg

The teaser for the final season of TCW also hints he knew what was about to happen. Although not sure he knew specifically about the clones being the ones to execute Order 66, as Dooku was the one organising all of that, even when Maul was Palpatine's apprentice.

You're right-
https://www.youtube.com/watch?v=ZLW2jkd6E7g

Very first bit of dialogue. He definitely knew about the larger plan

Maul knew lots about Palpatine's plans, but he didn't know every single little detail if I remember correctly, only Palpatine knew did.