The total diameter should be the same whether you are using an 18 or 19” tire. The sidewall will be wider on the 18” tire to make up the difference so you shouldn’t be concerned about lowering the seat height etc. Most enduro bikes use the 18” rear tires because the sidewall is wider. The wider sidewall better protects the rim from impact damage, which is a plus especially in rocky areas. 18” tires are typically heavier that a 19” tire making them a little harder to get spinning, a good thing when conditions are slippery, a bad thing when you are in the sand etc. I’m surprised that you are having troubles finding a 19” tire, that’s what most MX bikes are using. I’m guessing there aren’t a whole pile of MX bikes kicking around the Thialand country side though. Around here its 18” tires that are hard to find on the shelf.
David