Does 2 to 3 inches of compost and a layer of double-walled cardboard stop sycamore seeds growing through? It’s impossible to catch them all but I don’t want to cover them over if it will actually promote growth, not discourage it.
Sycamore seedlings (recently started) are annual weeds and die under mulch.
They are also easy to pull out, I pulled some yesterday so this is not a problem.