Meta products still have the most users though, so most companies/organizations still find value in them.
FediDB says the Fediverse has 1.2 million monthly active users (people that use it at least once per month). Bluesky has around 20 million. Threads, which AFAIK is the smallest Meta app, has around 320 million with 1 million new users signing up every day. It's on track to overtake Twitter.
Cross posting is definitely a good idea though! I'm not sure which tools handle cross posting across all the current platforms though. I'd love to learn if there's any good apps for it, since I want to more easily do the same thing myself.
You'd need some sort of fancy cutter to get them into the right shape, too. You could get die-cut stickers from a store like Stickermule for cheaper, but only if you buy in bulk (more than 100).
A Cricut would do it, and they are affordable. The bigger problem is that standard inkjet inks fade and even run if you subject them to enough water. You really want to use dye sublimation printing.