I rented a car from Avis, in Lampang and dropped at BKK airport. Very easy drive, never checked at all, but I had flight ticket, CoVid test result, app on phone with booking and travel
If by road, register at thethaiger.com/news/national/online-registration-site-up-for-interprovincial-travel-permission and it is easy, did it 3 days ago
tests are actually 2nd day and 8th day now. If 2nd day test negative then you can leave house after 5 days if no symptoms still have to take 8th day test. Kit ordered online and sent to home.