Advice on wiring a pool panel from a subpanel
All of your questions you have the correct answers, thhn is dual rated thwn so it can be used in conduit both inside and outside as required.

You do need an insulated ground. You can put in a 100 amp panel at the pool  and feed it with a 30 amp breaker or the size you want . 

the 1.5 hp 240 v motor is rated at 6 amps + 25% (motors require 125% of the FLA book values). 

I think you are on your way if you wanted another sub past the pool sub that is a sub in the garage fed from the house it is ok. As a detached area another rod would be needed for this sub.