If you requesting an answer for your question, i can help, otherwise there are a lot of other opinions above that may or may not help.
As regards Your Garmin Handheld.
I run Garmin Zumo System on my bike to navigate and i purchased a dedicated Thailand Map through
Garmin Showroom & Service Center
Sathorn Nakorn Tower, 100/38-39, 22nd Floor, North Sathon Road, Silom, Bangrak, Bangkok 10500
You can buy the Map on a small sd card that should plug into your handheld or you can either send your handheld to them or just drop in.
When you purchase the local map you will get a free update.
I have found this local addition to be very good, regularly updated and for exceed the Official garmin worldwide SE Asia maps.