I note you say "before European contact" but the examples you gave are from afterward. Lantakas came some time later, and the Burmese Dragon cannon was also 18th century, but Europeans were in the area well before this, with the Portuguese on the Malay Peninsula in the early 1500s and in Ayuttaya by 1511.

Chinese in Ayutthaya were already making firearms in 1500s. And while Ayuttaya had Portuguese soldiers at that time, in both Pegu (Burma) and Ayuttaya foundries were crafting firearms locally at this time. Chinese are credited with bringing cannons to Ayutthaya, prior to the heavy European trade of later centuries, but the Thais were doing quite well at the start of the 17th century. So well that Tokugawa Ieyasu imported Thai firearms to Japan.

The Portuguese were probably the earliest Europeans to Ayutthaya, in the early 1500s, and at that time, guns already existed in the region, although few in number. So while the height of Thai production of firearms wasn't prior to European contact, it wasn't the Europeans who brought the technology. In 1549 Pegu attacked Ayutthaya, and it was around that time that Ayutthaya started taking things like firearms more seriously. They'd existed before, but it was at this time that Ayutthaya made it more of a focus, eventually getting more of their own firearms from outside sources like the Portuguese.

As you probably know, the history of what's now Thailand and Burma is far from free of conflicts between the two regions. And at least since the 16th century, firearms have played an important role on both sides.

See also:

• Baker, Chris (2017) A History of Ayutthaya: Siam in the Early Modern World

• Tarling, Nicholas (1992) The Cambridge History of Southeast Asia vol 1