Using solid copper THERMOSTAT wire as speaker wire

18 gauge wire or any other gauge thermostatic wire (or other like fare) simply is very poor quality - non-OCC - low-fi or "no-fi" product never contemplated for audio, and simply cannot compete against quality audio speaker cables.... full stop.

The higher you go in equipment quality and resolution capabilities, the more graphic the unfavourable results.

That "assertion" IMO comes from a type that somehow sees himself as John the Baptist preaching in the wilderness and also somehow prides himself on throwing around nickels like they are manhole covers.

You have made a good investment into your system from what you have listed. Why scrimp now? Get a speaker wire that is in the same league as your equipment. Kinda like rebuilding Porsche with OEM or better components and putting re-cap tires on it save a few bucks.
Also in my experience with Bell or Thermostat wire is it is brittle and breaks easily, that is one of the reasons it is stapled in place to limit movement.

No use car jumper cables its sota.

People cannot hear the difference between cables for many reasons. It could be their system that is not resolving enough, could be their hearing instrument or just negative placebo effect since they often strongly believe, that cables cannot sound different. Often they feel defensive about inability to hear the difference (if I cannot hear, nobody can) and post questions about lamp cords or thermostat wires to find confirmation from the other people. Since wire is inexpensive just try it - If it sounds good to you then use the cheapest one, even if it is barb wire (be careful). No right or wrong here.

Hi Theo, I agree with your last post completely!, porsche with re-cap tires!, I tried posting my opinion on the door bell wire for speaker cables thread, when I noticed that there was some believers that thought cables are not a component, or that cables do not impact a system as much as a component, I exited the thread, cheers.