Bleaching a buzzcut/undercut?

I guess with hair this short it doesn't really matter for the hair if you wait, you wouldn't even notice if the short sides were fried. But you need to be careful with your scalp, bleaching hurts it and if you have a dry scalp, too much bleaching can make it irritated and even cause chemical burns.
For the long top you can just do 2 bleach jobs or bbs. In between full bleaching you should wait a month, for bbs a week.
With natural black hair you might have to bleach a LOT to get it to platinum blonde. Could be that you can't get your buzzed sides light enough before you have to cut them again. And regrowth might look weird. So I guess you should think about it again, if you really want the effort.

When I had the side of my head shaved the sides bleached so easily as the hair was short and near my scalp so there was a lot of heat coming from my head. You might find that the sides lighten up enough with one bleach.

Also I agree that it doesn't matter how damaged the sides are if you shave it regularly. I would put coconut oil on before you bleach to help protect your hair/ scalp.

Hmmm maybe I'll settle for just one bleach job.
This might be a stupid question but if its just one bleach job followed by toner for natural black hair, what is the end result color? brown-ish?
Also what toner should I get in this situation? any recommendations?

I decided to bleach my undercut this time around so I could dye it a pastel color, and it lightened up nicely.  I did have to use 30vol though, and my scalp is a little sore from it.  One round of 30 should do it, though - just be careful not to leave it on too long if it starts to feel uncomfortable.