If you do not report you will be fined 2,000 Baht next time you visit Immigration.
If you do not get a confirmation in a few days pop along to Immigration to avoid the fine.
Quite easy to understand and do.
400,000 in the bank OR 40,000 monthly income.
Easiest way is to put the 400,000 in the bank and forget about it
You can then extend your permision to stay every year.
You can either marry your girlfriend and go the marriage route.
Or as you say go the retirement route.
Having said that you never stated it was with a lady.