I've been meaning to make a post, but with the Ahsoka finale coming up, I guess I shouldn't put it off any longer.

I'll offer my opinion, and hopefully we can come to some sort of consensus on the matter.

What's considered a spoiler?

For Star Wars, I'd say that any movie, or a season of any TV show that's been broadcast to completion, is fair game to meme about without worrying about spoilers.
There's lots of stuff that I'm planning to watch 'one day', and I've sort of accepted that I might happen across a spoiler, without expecting the rest of the world to patiently wait for me to catch up.
For things that are currently airing, I think that anything that's already been established by the show's premise isn't a spoiler. From Ahsoka's first official trailer, we know about the following appearances: Ahsoka, Sabine, Mara, Huyang, Ezra and Chopper; a Night Sister, two mercenaries, and Thrawn. So I'd argue that a meme depicting a Night Sister offering a data file to Thrawn isn't a spoiler, even though we had two reports saying it was.
Spoilers for Ahsoka would include: anyone dying; cameo appearances from characters in the wider universe; big revelations, the ultimate resolution, or anything really that someone might think you're being a bit of dick about revealing.
Ideally, you'd wait a couple of weeks to post about these, but if you can't wait ...

How to post a meme with a spoiler?

We're limited by the current state of lemmy as to how to do this.

• NSFW'ing stuff isn't that good: most people leave it unticked so you're massively limiting your audience, and whether it blurs an image is user-configurable, so it might not even work.
• Spoiler tags aren't supported widely with mobile clients, and Connect doesn't acknowledge Markdown inside of them
• An image inside a text post is likely to get scrolled past, and some clients will grab the first image from the body and use it as a thumbnail.

So the best way for now is probably to upload a generic image for the thumbnail, and include your meme as an inline image in the body of post, with something in the title to indicate a potential spoiler. You can re-use the image I've used for this post if you like. It's at https://lemmy.world/pictrs/image/386b54a9-20d6-4ced-a9d6-a90f771b7171.webp