Will be fine, had similar setup (single flue about 5ft long total, stickup above roof of about 4 inches, aluminium plate maybe 24" square) No issues/signs of melting. TBH don't think i ever even saw a spark come out the top.
Twin wall flue will help with this but keep the inside of it clean(ish)! Especially if you burn a lot of wet or coniferous or treated wood. A chimney fire will probably fuck up your roof and terrify the pants off you too.