@Tijl *********
I think a lot of people think they can use the visa legally for 5 years when they only have activities of 6 months.
Of course the visa is only valid as long as the purpose of stay still is valid.
The OP specified a planned longer treatment, for as long as that treatment lasts I think Thailand is happy, not everything needs to be highly specialistic.
I think the people only having a relative short soft power activity who after ther activity still coming illegally in Thailand will be maore a reason to axe this visa or to implement more checks.