You sound like a targeted visa applicant. I wouldn't be saying that your purpose is to come and visit your girlfriend. Go ahead and apply. Be prepared to show your income and tax returns, and possibly a business if you set yourself up.
you can change the extension reason from marriage to retirement when the time comes, you'll need to have 800k in a Thai bank seasoned for 2 months or do the monthly method. I'm not sure about the nitty gritty funding details to make the transition, maybe
I'm not so sure, you may need to be on the particular visa or extension to use the funds for another extension, not sure you can change the extension reason and have funds transferred while on a completely different extension or visa qualify.
keyword here is renewed, you can stay as till your current marriage stamp permission to stay date, but of course you can't extend if the spouse has passed away.