Maybe using stopped dados with router, straight bit, and edge guide? Better making several cuts shallow then deeper. If straight bit not wide, can be adjusting edge guide and using two dado cuts. If round end problem, easy squaring up ends with marking knife and chisel.
Dado glue joint on 3 sides strong. Can also adding dowels inside dado, making stronger.
If dowels, dowel centers with points making easy marking end grain for second holes. Dowel centers good, dado or no dado. Also needing drill guide for straight drilling.