Not sure that a well insulated tank would be that useful. Unless you temper it very accurately - which is a pain to do - you will get a temperature change when you put chems in anyway. Tempering will be impossible from the outside (it's not even all that feasible using a normal plastic tank - takes too long), which pretty much means you need to prewet whether you want to or not.

The other option is estimating the expected temperature change, which is necessarily uncertain. You can always measure inside the tank with a thermometer to work out where you are, but then if you are wrong, how do you get the tank temperature to where it needs to be, if the tank is well insulated? A water bath will be ineffective.

Also I believe some proceses (fixing?) result in a bit of a temperature change by their nature, in which case you're stuck where you end up as a result of that. Could be wrong about that though.

Of course, this is all if you care about that level of precision. If you don't, then a plastic tank is pretty much good enough anyway.