“What would be good for fertilizing and boosting the asparagus?”
Asparagus benefits from a balanced fertilizer, so just a granular 10-10-10 is fine.
You’re leaving some of the stalks to go to flower, I hope? If you harvest it all, it’s not going through it’s ‘life cycle’ and that could be part of the problem. Leave one stalk alone from each crown and let that flower until it dies back of its own accord in the fall.
I don’t harvest any spears less than a pencil thick and several of them flowers and I had little red berries all over the place. Would well composted manure and wood ash work, too?
I know it takes a few years for the plants to become really well established. It’s just so frustrating waiting for that.
The new owners of our NY house are enjoying a fantastic asparagus crop. Glad for them but not for us.