Shallow mailbox post hole
A post set in the normal way but with only 200 mm depth won't be very secure.
Call the underground utility locating/marking service, if one exists in your area, and then dig your post holes carefully. It may turn out that the buried lines do not conflict with your plan or that your plan can be adjusted to work around them.
You can definitely trade depth of the post for mass of the footing. A concrete slab sitting on the surface (no ground penetration at all) will support a very large mailbox. For example, cluster mailboxes like those below are commonly bolted to a section of flatwork. (image credit: https://kentonbrothers.com/generalinfo/cluster-mailboxes/)

Your &quot;wide shallow concrete footings&quot; sounds similar to this idea. If a large concrete surface at ground level doesn't suit you then you could put it below ground. Excavate a large but shallow hole nearly to the buried cables, pour a concrete slab (or place a precast concrete pad), attach posts, and then replace soil over it so that only the posts rise above ground.