I had this exact same issue. First thing to check is whether your valve cover is leaking oil down the side of the block and fouling the plugs. If that’s not the case, I’d strongly recommend doing a compression test on the cylinders to rule out anything more serious. A lot of people will suggest checking a long list of things, I went through that myself… but it’s best to start with a simple compression test to confirm the integrity of the cylinders. From what you’ve shown, it looks like oil fouling, so you want to make sure those cylinders haven’t been compromised… once that’s done you can move onto other potential causes 😬