What is the best visa strategy for an upcoming trip to Thailand with a planned return after visiting Cambodia?

Hi, thanks in advance!

Flying into BKK, then flying out just short of 3 months later.

Planning on spending about 7-8 weeks in Thailand, then heading to Cambodia for a month and then heading back to BKK before leaving for UK again. (So will be in Thailand / BKK for a few days before the flight home)

Assume here the best plan here is to apply online for 60 day single entry tourist visa before I leave. I'll 'use this up' and then leave for Cambodia, and then when I come back into Thailand I assume I'll be on the standard 'no visa' standard 30 day tourist visa if I'm correct?

AIRLINE EXIT CHECK:

So the airline will check/require an 'departing' flight - to which the 3 month return will be valid (I assume as they don't care about visa this end) Airline don't know, nor care surely. They just want proof of exit at any time?

VISA:

The application for 60 day Thail visa WILL require exit date and proof, so I assume best bet here is to buy a BKK->Cambodia bus and use that as the proof I need?

Thanks in advance for this, much appreciated.