I’m currently buying an unfinished house and completely changing the floor plans. I used a web based service called upwork to find freelance architects and it’s pretty good. I am using an architect in Arminia and one in Ukraine. They charge $30 per hour and $25 per hour respectively. The lady in Arminia is good she has a PHD in architecture and I think she has worked on historical buildings although she has more modern buildings in her portfolio.
The upwork app does all the monitoring and billing and limits the working hours to 40 per week and in her case her first week she worked 14 hours.
If you know what you want you can send them pictures and measurements of the plot ( and some inspiration pics ) and a hand sketch then you work with them to refine what they produce.
As far as using the plans for approval in Thailand I think you may have to get the cad files and have a local draftsman translate them but I don’t know the area you are in and the requirements as a lot of places in Thailand don’t need serious plans. The local land office would have somebody to take your plans and make them compliant for a small fee.
I have used Thai architects and designers for several condo renovations in bangkok ( which is more expensive than countryside) and they usually want to charge a % or 500k plus. Depending on how long I use these ones on upwork I’ll spend between $1000, to $2000 each. I think for an entire house you may need to budget 3-5k but it really depends on how much you time you want to spend on it.
Check out upwork sand search for architect house plans or something like that. If you want the name of my Arminian architect ( she seems the better of the 2 i’m using ) then PM me.
Good luck.