What type of boat is it? I’m thinking that the load on the rudder of say a 36″ electric-powered warship isn’t going to equate to that of a D-Class OMRA racer.
I’ve used threaded 2mm dia steel rods for years and never had any obvious case of them bending under load, even with a couple of right-angle bends in them. Just keep the length of everything as short as possible to minimise the effect of any offset load and you should be OK.
Note that if you use two pushrods or thin wires to the tiller arm then the distance between the two holes on the servo disc into which you fit them must be exactly the same as that between the two corresponding attachment holes on the tiller. If they differ then the wires/rods will slacken or tighten up as the servo rotates.
Dave M