This has been posted before but the method keeps working. Even bellows I patched 10 years ago still function perfectly.
The key item is 3M polyester tape type #850 black. This tape is so thin, 0.05mm, that it does not bulk up the bellows even if you covered the whole thing. The tape is very flexible so it can be used to patch pinholes in the corners and then the bellows can be re-pleated easily. Best of all the adhesive is strong, doesn't bleed, doesn't creep, and never goes gooey.
The downside? It is an $$$ expensive roll of tape.
Another downside would be that I would have to order it overseas. Easier and cheaper to either make or buy new bellows. Downside of living in the outskirts of civilised Europe...