In my new garden I have now replaced the soil of about half the beds
with either manure, composted wood chips, or soil from my other garden
which was itself once 100% compost. The original soil was extreme sand.
Right now it is sandy loam and it will improve further.

I am mostly interested in water retention, because I like these $100
water bills less and less. With the other beds, when I change the soil,
should I go 100% compost or 50% compost-50% clay (which I can pick up
from my neighbor). In other words, is 100% compost always the best for
water, or given that the organic content is high anyway, go with some
clay as well? Obviously, no matter what I do, some sand will always be
in there as the earthworms churn and mix the various components.

In my other garden, by the way, I have seen that when a lump of clay
comes in (mixed with the manure) it does disappear in the soil in a
matter of 1-2 years, so adding clay in a loamy soil can be done.