Brad's advice looks good. Using other forums to learn what other people have done is always a good idea.
A bit of research on my idea of getting married in the Shengen zone could be worth it too (~14 months to go).
As it happens, the Thai masseur I've occasionally visited over the last couple of years was in the UK to "study English", with the visa having been arranged by his boyfriend. I gather he successfully had the visa renewed, however the Thai lad went home for a holiday & seems in no hurry to return.
Over the years, I've also visited several Thai masseurs in Berlin & every single one of them came over to live with a boyfriend several years earlier (& no longer lives with the bf). So it has been done in Germany.
Not advising anyone else here, but in the unlikely event that I were ever to do that, a pre-nup would be in place. Incidentally, a friend of mine was advised by his divorce lawyer to have a pre-cohab agreement in place before moving his prospective partner in. Whether that is good advice, or to generate more legal feees, I have no idea.