The standard Android keyboards such as Android Keyboard (AOSP) and Gboard do not display all the Thai characters.
Instead, half the Thai characters are not visible until the shift key is tapped.
This is a problem for people who do not have the key positions memorized.
Therefore a different keyboard is needed.
That keyboard is TSwipe-Pro.
TSwipe-Pro first swipable Thai keyboard
TSwipe-Pro has been around since 2010, and was the first keyboard to support swiping in the Thai language.
Originally it supported three languages: English, Japanese, and Thai.
Currently it supports English, Thai, Japanese, Sanish, Polish, and Chinese.
It has over 1m downloads and a rating of 4.2 from 10k reviews.
It is highly customizable with themes and also supports voice input.
TSwipe-Pro Settings and Configuration
There are a myraid of settings and configuration options with TSwipe-Pro.
The most basic are the Theme and various layout options.
It is really highly customizable and a very mature keyboard application.
First of course, set TSwipe-Pro as the default keyboard.