If you are getting close to the max, just leave them alone because if a shop that doesn't know what they are doing ruins them, you will have a tough time replacing them. The new linings will wear a pattern pretty quickly. We ran grooved brake drums 50 or 60 years ago and it worked then and it will work now.

I am especially cautious about cutting drums that have been used with non asbestos linings. Non asbestos linings cause hot spots that will cause the cutter to jump over them. I have a lathe that will cut 16 ½ x 7 drums, but I quit cutting them when all we could get was non asbestos. I rigged up a grinder attachment to grind the hot spots, but it is just not worth all the effort.