[toni-a]

Since you don't mind used items and you already own EF to Z adapter, why not consider Sigma 24-35f2.0, it is dirt cheap now on the used market and would give you the coverage you need. It's not small but it will replace several lenses.
I now your beloved 16-35 will be in the overlap area and this alone is major drawback, myself I found myself in the same issue on 7D2 with both 17-55f2.8 and 15-85 as options