Thailand's Army dominated government always needs to keep its junior Thai Navy partner happy. Interservice coalition maintenance is expressed in various ways including buying high priced armaments for the Navy - a carrier in the 1990s and 3 submarines agreed in 2017.
High priced armaments are there for a service's strategic need, pride/morale, professionalism, commissions and post-government career jobs (the last in Australia in spades...).
From 1992-1997 Thailand bought an aircraft carrier (the Chakri Naruebet ) from Spain. A carrier that Thailand couldn't afford (no fixed air-wing maintained for long), couldn't regularly justify and has never been able to permanently crew. Good for fleet reviews perhaps.
Thailand has been in a medium sized submarine acquisition for the Navy process since 2015. This is despite its most valuable territorial waters (the Gulf of Thailand with its approaches to the capital Bangkok) being too shallow for medium sized submarine operations.
In 2015-2017 Thailand purchased its first Chinese Yuan S26T submarine (for delivery by 2023 or so). There was Thai public opposition then over the cost and need for the first submarine. It was bought at a discount price from China which leaves Thailand obligated to China, to an extent.
Thailand's Government has now tentatively floated (in 2020) the idea of 2 more of the submarines from China - a 3 submarine deal really already made and costed with China in 2017. This again faces public opposition over the high cost (in 2020 particularly over "money could be better spent on anti-Covid measures").
But as the deal has already been made with China the Thai government's postponment in finalising purchase of submarines 2 and 3 is more for show to keep the Thai public happy than concrete.
Time will tell.