Landed at hotel, rang Chiang Mai wheels and asked for a car within the hour. The answer was yes they just needed to clean it. They arrived on time, 5000bht deposit, no credit card, 950bht/day for 5 seater auto (middle size). I needed to return it earlier due to flights, again rang them and asked they arrive in 40 mins which they did to the minute. No issues with deposit return.
Car rental can obviously be a lottery and on another day this might have been different but our experience was perfect.
Also driving in Chiang Mai is great, it not crazy it's just efficient use of the roads.