First off ,,, If you are going to stay in this buisness ,,,get a truck !
If you can't afford a wet saw ( at this time) the best way is to use the diamond blade in the grinder . For softer tiles ( 4x4 , 6x6 ) wall tile,,, then
just keep going with the roto-zip .
Worked for a few years without a truck or a wet saw . BELIEVE me,if you are steady working ,you CAN afford both of these and they DO PAY FOR THEMSELVES !