As you say Andy, though it is a "solder" first before compression fitting is not acceptable in industry, basis being the tin/lead type solder will cold flow under compression, post solder again on a compressed fitting can be used, as it fills the voids and prevents cold flow as such. Crimped bootlace or ferrule first on fly wire, then under screw compression application. Alleviates the well pointed out attachment fatigue consequence of rigid solder to wire union.
As for soldering wires to thermoset plastics, soldering iron plays the most significant factor, mains powered soldering irons are going to cause problems. have 6 soldering irons for different tasks here, soldering 1m/m smd parts to large copper layers which draw heat away form the iron. Technique is all, takes skill soldering onto thin flexible plastic "ribbon" like interconnects.
Eutectic temperature of usual grade 60/40 roll solder is 240C, that is a minimum, around 340C for the actual iron tip temperature prior to application to the parts to be soldered. HTH j.