Can you explain what is a thru-keel windlass and maybe post a link to it?
Windlasses are installed on the bow area, either together with anchor in a holder/roller on deck or windlass is belowbow deck under a hatch and anchor exit thru a hole on bow.
If you don't have the windlass cover hatch then you should install the windlass on deck and use a bow roller who can lock the anchor in place.then you need to drill the small holes for the windlass and a bigger hole for the chain dropping down in anchor locker.
Your 257 Advance has a anchor holder but it's made for manual recovery and storage and from th photos i found i don't see enough space for a on-deck windlass and installing one below deck would be a hefty fibreglass job to make a box with a sturdy basement and cutting a big hole for a access hatch.
Much more simple and definitively much less expensive is the use of the old school Anchor Ball for retrieving a anchor, but you need space for the retrieve maneuver, nothing for the sand bar or crowded places.
Chris