Can I renew my 12-month Non-O multi-entry visa in Thailand, or do I need to return to my home country?

it is not. 😕

Inside thailand on a year-long, multi-entry Non-O visa <- the kind that gives you a 90 day stamp every time you enter, your options are;

1 - get a yearly extension by meeting the financial requirements based on the reason you want an extension

OR

2 - get a 60 day visit thai family extension if you're married to a thai or have thai children.

Now it is possible to go to a thai consulate in a nearby country and get a year-long, multi-entry Non-O visa without any proof of funds, but only if it's based on marriage to a thai and only at two thai consulates in the area the one in HCMC Vietnam and the one in Savannakhet Lao.