I can't suggest anything on the toner transfer method, never having tried that, but can suggest an alternative etching solution that works at room temperature and is very clean. The solution can be used many times, by replenishing its hydrogen peroxide, until it gets saturated with copper. I keep the same solution until too much hydrogen peroxide or acid is neded to keep it etching - this is a sign of copper saturation.
This is the etching formula suggested by Contact Chemie, that used to produce the Positiv20 brand of spray photoresist. It should not attack permanent ink like that from Sharpies. I used to make corrections using permanent markers.
The formula goes as:
330 ml. (cc) concentrated hydrochloric acid (%36 I guess - not sure)
70 ml. hydrogen peroxide (%35)
the rest water to make 1 liter (1000ml. or cc).
I don't know the formula in fluid ounces, etc.. I don't know if you are accustomed to the metric units ml. or cc..
If you use more acid and peroxide, the etching time gets shorter but the solution becomes somewhat unstable - the first one I made had around 250 cc. hydrogen peroxide, it kept blowing the cap off of the bottle
The etching time should be less than 20 minutes with the standard solution, probably less than 10 minutes as well, but I am not sure. I have not used this since many years, nowadays we use a professional system at work - produced by Think&Tinker USA. This setup works using a sulphuric acid plus peroxide etchant.
When the used solution starts getting dark during etching, a little hydrogen peroxide is added and the process continues.