Is there any way to estimate the weight difference between a trimmable horizontal stabilizer and a fixed tailplane?
Hard to find a general reference for this one. You're adding the weight of the hinge, the actuator, and the supporting structure now connecting only to the hinge axis.

Torenbeek section 8.4.1 gives a weight estimation for preliminary design purposes for the tail group of high subsonic aircraft: it mentions a weight increase of 10 % over the weight of a fixed horizontal tailplane.