Yeah, for sure the first thing is to choose between some kind of spin-off (you can keep all revelations) or a movie meant to be watched before the OT (then Save the reveal).
An idea I had some time ago about this, to strengthen the concept of Anakin being dead without editing Ben’s words in Episode IV would be to use General Grievous’s chat with hologram Palpy. I don’t know if it’s doable, but…

As you said you could hint that Anakin is dead after the duel on Mustafar by removing him with Vader’s suit.
You could remove any mention of Anakin being Obi Wan’s apprentice and Anakin calling him “master”.
You could remove Grievous’ arrival on Utapau (keep the mystery until Obi Wan’s arrival on the planet). Then (and here comes the hard thing) you could cut the duel between Kenobi and Grievous and keep the general alive. Then you place the hologram chat with Palpatine in the third act, and replace the “But the loss of Count Dooku” with “But the loss of Skywalker”, to hint that Anakin is dead. Finally Palpatine replies “Soon I will have a new apprentice, one far more powerful”.