Nice build and great job for your first project. A couple things I might add is the broken bits are usually from not holding drill straight in and straight out. Also using a countersink bit can allow you to hide the screws relatively easy. Also a deck of cards are a great way to perfectly level a stand. Also when using the water based poly applying a couple coats and then sanding with 220 paper until hazy and smooth then reapplying another coat will lend a nice smooth coat. Also warping down with a rack cloth after sanding will avoid those little dimples you were getting in the finish. Here’s a stand I built very similar to yours.