The stringy stuff is likely the pus/infection. Sometimes there is not a kernel, it's stringy or ribbony and can branch off into the foot. Those are much harder to clean out. Sometimes it's a combination of both stringy and a plug/kernel deeper in the foot. I've currently got a rooster with an infection like this that I've been treating for quite some time and it keeps recurring. I may eventually have to cull him, but will continue to try until it's hopeless. I will attach some links that may help at the bottom of this. Every case is a bit different, some are simple, some less so. First link does discuss the stringy type. If there is any secondary infection from the surgery/treatment I use injectable penicillin, but on it's own that will not get rid of the bumblefoot, the foot still has to be cleaned out.
http://www.the-chicken-chick.com/bumblefoot-causes-treatment-warning/
https://ouroneacrefarm.com/bumblefoot-treatment-tricideneo/
https://www.tillysnest.com/2015/12/non-surgical-bumblefoot-treatment.html/?spref=pi
http://www.the-chicken-chick.com/bumblefoot-causes-treatment-warning/
https://ouroneacrefarm.com/bumblefoot-treatment-tricideneo/
https://www.tillysnest.com/2015/12/non-surgical-bumblefoot-treatment.html/?spref=pi