Is that rent pre-covid? I have an offer to move to rent another property for 10k/month, they were renting it 18k/month pre-covid, if I pay rent one year in advance. The rents are down in Samui, that's if the landlords are lucky to get renters at all.
I know only one guy who has long lease, but he leased the land and built a house on it. I think it is a 30 years lease, and then the landlord gets the house and the land back. However with ever changing immigration requirements I don't see many foreigners willing to commit on 10+ years leases, as if the rules change they may need to go and lose their rent.
From your description you probably have a townhouse, and there are tons of these on the market, my Thai gf is looking to buy property, and she has seen heaps of them in the last 2 months.