Paid ads should not only need to be marked, but noticeably different in a timeline. Something obvious like a different post color.
Twitter fits ads in the middle of content and just puts a little tiny "Ad" in the upper corner (on mobile at least) and at a glance scrolling through you can't tell it's an ad, other than all of their ads now being for some shady mobile game that lies about how it looks or crypto in various forms. Those should be required to have a different color background than actual user posts, not just a size 8 font "Ad" in the corner of the post on a 3.5" screen.
In fact, let's make it impossible to implement well, let's take a page out of the NHTSA handbook and require the "Ad" text to be a specific real world size like they do with the car warning lights. Make them figure out what size it needs to be for various screen sizes and display DPI if they want to shove ads in the middle of content like it was user posts.