The bus is a direct long-distance option from Bangkok, but it trades a hotel night for many hours seated. Compare the whole journey with a sleeper train or flight before booking.
The Transport Company identifies Bangkok–Chiang Mai as route 18 and publishes services from Mo Chit 2. Departure times, vehicle classes and fares change, so confirm them in the official booking system for your date.
Mo Chit 2 is not the BTS station of the same nickname. Leave enough time for the road transfer to the bus terminal, especially before a holiday.
An overnight bus may save daytime but does not guarantee useful sleep. Check seat layout, arrival time and the Chiang Mai terminal. The Arcade terminals are east of the old city, so most visitors still need a local transfer.
During northern haze season, check current air-quality information before treating Chiang Mai as the automatic next stop.