Maybe better to find a lightweight hardtail MTB that suits your requirements & replace the fork with a carbon fibre fork with no suspension. Buying separate components here is expensive compared to buying a complete bike. I bought a MTB here for not much more than the price of buying the Rockshox fork separately.
I know someone who built a gravel bike using a carbonda frame from China.
https://www.carbonda.com/road/gravel/cfr505.html
He spent about 100,000 baht, but if you want flat bars the shifters & brakes would be a lot cheaper than those for drop handlebars.