@Dimitris *******
There are different types of Non-immigrant visas. There are several Non O visas for various purposes. It would take too long to explain them all.
For long stay without working if over 50 then most people get either a Non O-A visa or a Non O visa ‘for retirement’.
You can only get a Non O-A in your home country.
You can get a Non O either in your home country (but some don’t offer it) or within Thailand.
In recent years they have introduced compulsory Health Insurance for Non O-A visas and look as if they are increasing the bank balance required.
For these reasons it is now better to get the Non O.
If you have more money there are other options as well. Non O-X or Elite but few people get these.