No one can tell you whether $60/hr is fair because we do not have access to your overhead expense, cost of living, desired profit, etc. I can tell you the biggest mistake you could make is to undervalue your time. I can understand throwing in a freebie or few man hours at the end of a season here or there only for the very best customers and only on your terms after they have given you 5 figures in a season. However even if you tell them this straight they will not appreciate it like you intend. What essentially happens is that you have undervalued your time and they will forget the savings the very next time you hand them a contract renewal with price increase. If you do this to anyone on a regular basis you have shot yourself in the foot because it will become very difficult to raise prices later and anyone they refer will expect similar pricing.
Your setup is enough to support additional laborers. Please understand that if you expect to charge half the amount to have double the man power you have again done yourself wrong. Generally having extra laborers does not decrease your rate. The only people who can make this happen are large scale and are using reduced rates to make it up in volume. I can not imagine that legit organizations are working for $20-30/hr anywhere. My cost to be on your property is greater than that per man. There is one company around here that intentionally lowballs other contracts when competing straight labor by offering $35/man hr. They do large volume, but they make up for it by overcharging the customer for bad material. IE charging inflated pricing for recycling center mulch and then arguing it is premium grade. They have gotten a few of my customers once or twice, and I see them all the time when we are dropping off our garbage brush, pallets, poison ivy rip outs, rotten stumps, etc.