5Z schrieb:

Is it OK to fly a full aerobatic program with the K21
having the spinn weights attached to the tail boom?

Not according to the addendum page for flying with spin weights - only
spins are allowed.

I've seen both: One Handbook forbids aerobatics with tail weight (which
makes absolutely no sense), the other allows it (or rather just doesn't
mention it.) Many people do it.

You may want to contact Schleicher for the proper guidance on this.

This is never a bad idea.