We first hear of the Emperor's mysterious return from death in the opening crawl. Sidious has made a public transmission which has terrified the galaxy.
Now I'm aware of the well-established principle that the villain has to give a speech announcing his fiendish plans in advance to all and sundry - it's an essential part of how any self-respecting evil person tries to take over the world.
I'm also aware that for Sidious's original plan to work that he had to draw out Kylo Ren to Exegol, which means revealing himself to some people at least.
However, he didn't have to make an announcement to the whole freaking galaxy. Why throw away the element of surprise? We see that the Resistance are able to discuss the transmission and formulate a plan about how to confront him, which ultimately leads to the destruction of the whole fleet of Star Destroyers.
His strategy would undoubtedly have been more effective if nobody knew he was alive apart from Ren (and perhaps a few trusted First Order generals). That, after all, was the default position - everyone assumed he'd died when the Second Death Star was destroyed.
Did he have a reason for throwing away the advantage of surprise and making his return public?