You know what to do now. Get pre approved for the most you’d ever consider paying and make an offer the first day if it looks good, telling them not to sell without giving you their best offer to beat. If you show you are approved for 500k and they get a full prixe 400k offer from you, they would be tempted to at least see if you will go higher.

Your agent failed you letting a house get away like that. I doubt the sellers were in such a hurry they would not have waited a day or two for you to make an offer if you expressed your interest aggressively. Make sure you are reviewing every new listing in your target area every day.