After years of trying several materials and system, I use now only stainless steel labels. Size 75 x 35 mm. Ordered some 500 to a metalworkshop with a laser cutting machine. Welding wire of diameter 1mm or 0.6mm to fix. Also some left overs from stainless cables can do the job, when split into reasonable thickness.
The text is stamped with a set of ABC letters from 3 mm height and nowadays also sometimes 4mm to highlight capitals. I have also 3 and 4 mm numbers.
With small trees I hang simple with a loop the label on a branch or stake (also for bushes) That the wood grows over the wire is no problem as long as it is stainless.
When trees are older I drive the u hook into the bark with an old screwdriver. Later on it overgrows nicely and the wire sticks beautifully out, with the hanging label.
Hereby some just made new ones:
Leo