I got a FUE hair transplant 21 days ago (4500 follicles). About 70% of transplanted hairs have shedded and others are shedding

It is normal to shed transplanted hairs weeks after surgery.  The new hairs may take 6 to 12 months to grow.  You can trim your hair.

3 weeks after a FUE surgery the grafts are in place and will not be affected by buzzing your head,
Dr. Robert Jones

At three weeks after transplant, it should be safe to trim the hair to "zero" without risking damage to the grafts.

These hairs will sooner or later shed. They are never a problem for new hairs that start to grow. Usually, there is a lag time between the time the hairs shed (2-8 weeks) and the time the new hair grows (3-8 months). IF for any reason, there is an overlap between the two, pay no attention to it for when the new hair grows, the old hairs from the grafts that may persist beyond 8 weeks, will eventually shed