What you describe does not sound like what runners and cyclists can a ‘bonk’. A bonk will bring you to a walk, you balance will be messed up, your cognitive function slowed greatly, you’ll be ravenously hungry for sugar. Nothing a couple of candy bars and a Coke won’t fix. In fact after eating these things after a major bonk I was back to good form in the race within minutes.
If you were at a HR of high 150s you were in Z3 and burning a lot of carbs. When you dropped to low 140s you went back into a fat burning mode. The more Z1-2 base training you do the less this will be an issue. Then when your AeT is high 150s you’ll still be in Z2 burning fat but moving much faster.