Be advised that since 16 July, you have to be on 12 month extension of permit of stay based on Non-Immigrant O visa in order to get the 5 year extension. In case you are still on initial 90 days permit of stay, only 2 years extension will be granted. This is according to my own experience last week.
The stamp indicates the admitted length of stay (here 90 days) based on the single entry Non-O visa. The red stamp itself is NOT a visa; the visa itself has changed its status to USED and is not valid anymore. By the way, I got exactly the same red admission stamp and handwritten remark but with 1 day earlier because I entered Thailand on 11 July.
I arranged many German visa invitations successfully and none of the Thai visitors were "employed", no employment contract, no holiday letter, ... They lived in a rural area with the typical agriculture-related jobs. And some of them are elder--already in pension receiving age.
Strange. My local DLT extended my Thai Driving Licence having either a Tourist Visa or Non-Immigrant O Visa (NOT the extension based on that) each time without problems.
Concerning Thailand, tax residency is defined solely by the total number of days a visitor is staying in Thailand per year. If living less than 180 days, he/she is not a Thai tax resident.