All Hanlin V3 firmware's will work on the EZ Reader Basic. The one that is most different from the EZ Reader standard version (and therefore the one that is most worth trying) is LBook. I think it has dictionary lookup at least for some formats (e.g. FB2). I don't know if it supports Thai.
In ePub, you can provide your own fonts if you are willing to modify the ePub, e.g. with Calibre (so it has to be DRM-free). See
Fonts and Epub - What works on Sony, Works for Pocket Pro too!
The other option is OpenInkPot (
release candidate 0.2-rc1). It uses FBReader as its primary Reader (it also has CoolReader3), and you can try this on a Desktop machine to see if it is likely to work for your ebooks. Since it changes the filesystem type for the internal memory, switching back to a standard firmware is a bit more involved if you try OpenInkPot and don't like it. Also, make sure you backup the internale memory before installing OpenInkPot.