Not sure the exact size of most of your lawns but I would add either a small zero turn rider 48 to 52 inch cut or possibly even better choice would be a stand on unit. Solo or employees either way your production will increase.
10 x $25=$ 250 a day
5 days x $250=$1,250 a week
4 weeks x $1,250=$5,000 a month
Adding new mower
12 x $25=$ 300 a day
5 days x $300=$1,500 a week
4 weeks x $1,500=$6,000 a month
This is just a example solo without taxes and expenses. 2 lawns is a example but honestly think 3 or 4 more lawns a day would be more accurate.