I'm putting together a photobook using one of the numerous, popular sites for this. However, no company supports non-ascii characters in their text tool.

So, I'm creating a transparent image with just text in GIMP. However, as soon as I dump the image into the photo book editor, the text looks ok at first, but scaling to 200% viewing, it's clearly getting distorted and I worry the actual printing won't hold up:

https://imgur.com/a/tcrx3

I want to maintain really sharp text because some of the details are clearly getting lost which are pretty important with the Thai language.

I keep increasing the image parameters to get something that looks sharp in a really big and then scaling down from that but it's not working - for the text box, I'm at 10" x 4" canvas size with 1,000 ppi with 150 px font size... The image in the screenshot is a 292KB gif image using those dimensions which I would hope would have been sufficient, but it's clearly not.

Any suggestions you can offer for settings?

Using one of the on-line providers for the photobook? Some have an off-line application but the same comments apply. It is just a representation of the final product. Might be good, might be awful. All you can do is provide as good images as possible.

This example using 20 cm x 15 cm photobook template. In Gimp that is 2362 x 1772 pixels @ 300 ppi. Gimp is a raster editor, uses pixels and 300 ppi is generally accepted as suitable for quality printing.

You could incorporate the text into the image itself. Add depth and then the text. If you use png format then transparency is maintained. Not jpeg, it loses transparency and introduces artefacts to text.

However from your post the text is separate to retain the background. Again use png format with transparency. Gif will look very 'blocky / jagged'.

Make the text a suitable size for your Photobook. Any scaling on site will degrade the image. The example text is 1800x400 pix for the 20x15 cm book format. Font does matter, keep it simple.

Back to the comment at the top, the on-line preview does look horrible. Provided the printer does not scramble anything (no guarantee there) the print should be ok.

example: https://i.imgur.com/4VuiWhF.jpg

You could ask the printer for a sample. Unlikely to get one, otherwise check reviews for customer comments