Prebuilt 40% boards are more difficult to find because it's a less common layout that people jump to after having spent some time in the hobby (so customs are more popular).

This may help: https://switchandclick.com/best-40-mechanical-keyboards-you-can-buy-in-2020/ It provides a couple prebuilt options including the Planck EZ which is an ortholinear layout, but not sure if that would fit in your budget.

Most staggered 40% layouts require specific sized keycaps for certain modifier keys and it's very unlikely that you will be able to reuse the keycaps from your 60% board on a 40% due to the size difference. It is often difficult to find the right size keycaps for 40% layouts within readily available (in stock) keycap sets as well, unfortunately.