Given the short build time you have, an M20 bolt makes sense. Bronze bearings are meant to run on precision ground & polished rods but they will run on anything vaguely round with a shorter lifespan. As long as you have some spares and oil the bearings regularly, they should last long enough. If possible, wait until you have the bearings before buying the bolt; take the bearing along and check to see how it fits on the bolt and how smooth the bolt's surface is. I use bronze bearings running on M8 bolts for a lower speed application and they work very well after I polished the bolts a bit.