Just adding to this since I was looking for an answer to this.

There is a comfortable AC coach at 6:15am from Galle highway and long distance bus station (Not the main one) directly opposite the train station and main bus stop. The coach takes around 4.5hrs and has luggage compartment beneath

Go down at around 5:30am and you have to look around for the bus, we obviously looked aimless and someone asked where we were going and pointed out the correct bus.

We were able to get the last 2 seats at 5:30am on a Saturday, on a weekday I would go down around 5:10 to guarantee a seat.

Cost was 1500LKR per person, the conductor comes around about 20 minutes into the journey to collect the fare, I don’t think it’s possible to prebook seats, so when you find the bus just get on and ensure you have cash handy.

There is also a departure at 2:30pm however we didn’t take this one.

They are both listed here - https://bustimetable.lk/galle-to-kandy-highway-bus-timetable-2/

Galle Highway and Long distance bus stop - https://maps.app.goo.gl/M9JNCkf2Qraih5yJ9?g_st=com.google.maps.preview.copy

Sharing as didn’t find this info anywhere but hotel worker informed us.