The math
Bosses respawn on a 6-hour internal server clock and drop eggs that take 15 real-world hours to hatch. The 9-hour gap means you can never catch up by skipping a session.
Cycle alignment
Run Verdantstag at the top of every session, drop the Bamboo Egg immediately into the incubator, then run the rest of your route. Repeat on the 6-hour boundary.
Hourglass policy
Save Hourglasses for Verdantstag drop windows unless you have a stockpile. Never use them on filler Grass or Shadow eggs unless your hatch queue is empty.