You may already know this, but often different immigration offices have slightly different requirements. It’s always worth asking for a list. They have lists. 😊
They can also advise you how to get any kind of different visas/extensions you might be thinking of.
If you speak to them directly you won’t be wondering what’s going to happen at the ‘big bad immigration office’ or be convinced by agents that it’s all so difficult so you need to pay them to do it for you.
to clear up any confusion, rather than going to agents, it might be best to go to the immigration office you might use for an extension and ask them. They won’t bite you.
Your last paragraph mentions frustration that things are not consistently applied. The entire country is structured like that. It’s not done accidentally either.
It’s something anyone living in Thailand will need to get used to.