To OP: I have been grinding for over 5 years. A brief summary - Residential grinding requires a dedicated machine. IMO, the minimum machine is: diesel only, 50 - 60 HP minimum, 4WD (look at RG-66). If you do the math on the inch method, the bigger the stump, the less you make. A 36' stump is closer to 5 - 18"ers than it is 2 in cubic volume. I do not remove chips. Jobs are bid to achieve the price per hour where I can pay the bills and have something left over. I seldom do locates. I'm grinding 4-6" deep. Utilities are at 24 and 36". It is a machine of destruction. The customer understands when I "find" something, usually it is too late. I'm not responsible for things they did not know were there. Sprinklers, phone lines, drainage pipes. There is a lot of maintenance to a grinder. If you have any questions feel free to PM me.
Your world is different than mine, we have almost zero natural rock. Your tooth life will be shorter than mine.