I’m sure making Sifo-Dyas a Sith apprentice was one of George’s ideas at one point, but both the original EU and Clone Wars season 6 (which was technically released under Disney but written with George’s direct involvement) officially established him as a well-intentioned Jedi whose drastic actions were discovered and exploited by Sidious, who eventually ordered Dooku to have him killed.
Definitely an intriguing idea for a fanedit linking the trilogies, though.
I honestly think there was more to the story than just what was established in Clone Wars. I’m reminded of this retrospect. Dave Filoni really implied there was more going on.
My theory is Sifo-Dyas and Dooku were both secretly trained by Palpatine at the same time. They were both challenging each other for who would lead the Separatist Movement. However once Sifo-Dyas no longer served a purpose Palpatine had Sifo-Dyas killed as he fulfilled his requirement of ordering the army and the trace of Sith’s involvement was lost due to the illusion it was a Jedi who set things into motion. So sort of what you’re saying but with the added layer of what George talked about.
I think this has a lot of potential and could be incorporated into a fan edit of TPM by changing Sidious Hologram to Sifo-Dyas (played and voiced by a new actor).