In a 25X25 garden? that would cost a fortune. The lumps do dissipate,
and there are ways to make them dissipate faster (like, planting
veggies with tough roots in those beds the first year). Basically, it
boils down to whether a mixed loam holds water better than sand and
compost alone. There are also veggies that dislike sandy soil, that I
could then grow more easily.

Holding water is probably a complex problem. There are micropores,
which retain water if the surface tension somehow makes it stay there
(that is, if they are not too big pores). And organic matter does
absorb water, though for how long I do not know.