I have always heard that Wyandottes start to lay later than other breeds (I saw you have SLWs), but my SLW was the 4th out of 10 chickens to lay. My GLW is NO WHERE near laying still. YIKES!
I started mixing chick starter with layer crumbles at about 16 or so weeks, but mine don't really eat a lot of "chicken feed" - they eat chicken scraps (like yours do
None of mine started laying until my egg-machine white leghorn started laying at about 20 ish weeks. Then I think the RIRs saw what she was doing and COULDN'T be outdone. Then one of the BOs started laying, then the SLW, and then the other BO. I actually forget how old they are now, but they were hatched April 6th, so I am guessing around 24 weeks now.
Unfortunately, I think you probably still have a little while to go.