Book your return around 55 to 58 days from entry date
The day of entry is counted as day 1 of your 60 days..
This ensures you don't need to pay for a visa extension for 1 or 2 days or pay the overstay fine if you don't get a last minute extension..
In case of flight cancelation or schedule changes.
Check with the airline if it's possible to change the ticket booked with your airline miles just in case you decide to stay longer and extend your initial 60 day excemption entry
There is also an Evisa option for Cambodia so you can apply online before you travel which speeds up the entry process as you don't need to do the visa on arrival when you get there.
Generally many insurance companies don't cover pre existing conditions or if you declare these conditions there is an additional premium and limited coverage..
Depends entirely on your individual circumstances, age, financial situation, income from external sources outside Thailand (if any) so quite a range of things to consider.