I don't mean to alarm you. I had one hen that seemed to constantly get open wounds on her feet this past summer. She died suddenly a few weeks ago. They found a circulatory/ liver congenital problem in her. While she was alive I couldn't find the cause/source of her bumbles at all. Soft bedding in pen, soft bedding in sleeping houses, no sticker bushes, no glass in their free range area etc. It turns out that they weren't bumbles. They were a sign that her liver was dying and her circulation was very poor.
If you can't find a source for the bumbles or if a vet says they aren't bumbles, it might be time to have an overall health check done. It may help pinpoint hidden/silent problems. Ducks can seem A-OK while they're having serious health problems.
My vet never recommends home surgery or squeezing the bumbles. He's an exotic animal specialist although my ducks have been the first he's seen professionally.
I use a hydrogen peroxide soak and one drop of iodine on the bumble. Only use the iodine every 7-10 days because it is absorbed into the tissues.
I also use manuka honey & coconut oil to make a wound pack to cover the bumble. The manuka honey is a gentle peroxide. I use the highest quality medical grade i could afford.
I've had some success using hydrocolloid bandages (pimple bandages) to cover open bumbles. They encourage wound draining and are somewhat waterproof. I've never used them on bumbles with hard scabs.. only new open wounds.