It depends on how big the testicles are. If they are mini goats it may be doable. If they are standard size goats probably not. I have banded some fairly well developed bucklings though. Pull the scrotum through the band first and then work the testicles through one at a time. Be sure to give a tetanus shot. It is very important that you get BOTH testicles.
Just a comment. You know how well developed your bucklings are and I don't. However, if they can be banded the discomfort only lasts a couple hours or so, if that, At least that has been my experience.
I've had a vet castrate a few by sedation and cutting. It wasn't any less painful for the goats. So no matter how it's done they are gonna be sore for a while and you will need to watch for infections.
Banding in my experience is the best way to go. My little Buck was 6 months old and I had the vet do the procedure. Levi didn't act like anything even happened. Pain meds and local freezing were given. They fell off in 7 weeks with a nice smooth clean underside.