Can I start paperwork for a one-year marriage visa in Thailand before my 90-day non-O visa is up?

Yes get your 400k in the bank for two months prior and go do it

As posters correctly stated at most offices you can file for an extension of stay when you have 30 days or less left on your current stamp and a few offices (Bangkok, Chiang Mai, Buriram Samut Prakan, Phuket, and others) allow you to do it when you have 45 days or less left on your stamp

What you should do is go to your immigration office and get the hand out listing the requirements needed at that specific office for the marriage extension and while you're there ask them what's the earliest you can file for the year extension

You can apply for a 12 month extension with either 45 or 30 days remaining on your entry stamp, it depends on which Immigration Office you will be using. Do you already have the 400k in a Thai bank account in your name because, if not, it needs to be seasoned for 2 months prior to your application?

All offices allow up to 30 days early, some allow up to 45 days early.