Hi, @Promphan Suksumek. Interesting project, and nice to see OT1.8 variable fonts being used.
One comment about the variation implementation: you have used the 'wght' axis with values from -1000 to +1000. You should change to a custom axis tag, such as 'EMOT'. There are two problems with using 'wght' as you have: the valid numeric range for the 'wght' axis is 1 to 1000, and the variation in your font is not weight! See Registered design-variation axis tag: 'wght' for a description of the 'wght' axis. A custom axis tag can be anything you choose so long as it uses uppercase letters (details on syntax here).
Software that supports variable fonts is likely to support whatever axes are in a given font, custom or registered, in terms of exposing in UI and ability to select and use variants.
But software is not going to know what a custom axis is doing, and so cannot do anything with it beyond allowing manual selection of an axis value by the author. Even if two fonts have custom axes using the same axis tag, software can't assume there is any connection between them, so if you switch from one to another any setting for custom axes will likely be cleared and need to be redone.
A registered axis potentially could specify semantic details, such as a requirement that all glyphs should or must change uniformly in a particular way.
The 'slnt' axis to some extent makes such an assumption, though it's not stated as an explicit requirement. And 'wdth' does state an expectation, though it doesn't enforce a precise metric. For 'wght', though, there is far too much history to impose any new constraint now.