@Stuart ********
I am sorry but 'Thai Legal International" advised me a Non Immigrant B visa...I am pretty sure they know what they are talking about...
I quote: "As for your visa: since you're not considering legal marriage at this stage, then — the Non-Immigrant B Visa (Working Visa) — is the most appropriate route for your situation."
And:
"Key requirements include:
An invitation letter from the Thai school, stating the position (volunteer English teacher) and duration
School registration documents
A copy of the director’s Thai ID
Possibly your TEFL certificate (once completed) and your CV
Proof of residence in the Netherlands
Health insurance covering your stay in Thailand
Completed the visa application form and paid of visa fee"
Turns out, I also had to obtain a Certificate of Criminal Past in my country...