In terms of the Order 66 Arc, the only thing you’d have to consider is Fives is in the Umbara Arc, but besides that I really think you could put the Order 66 Arc anywhere you want to break up the action with slower intrigue. Just an idea, but would it work as a “season premiere” for your Season 5? One reason that might work well is after Ahsoka leaves the Jedi Order it would slow down a bit to set up the dominoes for the last few arcs (which tie everything together) and then escalate to the finish line. Another alternative is to make it in Season 4 before Ashoka’s Fall which would fit with the theme of deception (things aren’t always what they seem) and betrayal that we see. That would make Season 4 kind of clone heavy though.