First of all, it is a good idea to calculate the transmission spectrum and the selfconsistent part as two seperate calculation, as you will avoid to perform the selfconsistent calculation again, if you want to adjust the transmission spectrum parameters.
When that is being set, based on your input, i would go for a smaller basis set, and perhaps increase the diagonal mixing parameter to a slightly higher value - like 0.2.
However the missing information is - how much time is spent in the calculation of the transmission spectrum versus the selfconsistent part?