I wish to extend my Non-O marriage visa by 30 days. Do I need to bring a departure card with me to immigration? (I have the arrival card PDF - do I need to book departure flight to get a departure card, if needed?)

You're confused a bit 😮 

First there is not 30 day extension on a 90 day Non-O visa based on marriage 😕 

You can either get the one year extension by meeting the proof of funds, turning in the required documentation etc 
OR 
You can get one 60 day visit thai family extension (not a marriage extension a visit family one) 

That's all that is available on a 90 day Non-O visa entry 

Now as far as your question;

When they used to give out the old paper two part arrival/departure card you used to have to make a copy of the departure part and include it with your application 
 

They discontinued those paper arrival departure cards years ago. Just leave that space blank on the TM7 application for extension. 

You also don't need a copy of the TDAC (Thailand Digital Arrival Card)

it's NOT a requirement when you apply for an extension inside the country at any immigration office 

This is the clause of the Police Order that allows you to get one 60 day extension based on visiting thai family (and in your case your thai family happens to be your thai wife) 

As I said, if you're not getting the year extension based on marriage this is the only extension you can get on a marriage Non-O visa 

I received a 30-day extension on a Non-O marriage visa last year, followed by a 60-day extension. I originally applied for a 60-day extension at the time and the immigration officer said I could get 30 days followed by 60. (Which I did)

8 minutes ago, Zenkman said:

I received a 30-day extension on a Non-O marriage visa last year, followed by a 60-day extension. I originally applied for a 60-day extension at the time and the immigration officer said I could get 30 days followed by 60. (Which I did)

No, you did not.

You entered as a tourist with either a tourist visa or visa exempt, and then you got the 30 day tourist extension, and THEN you got the 60 day visiting Thai family extension.

There is no such thing as a 30 day extension to a non-O visa.

A peek in your passport should be able to jog your memory.

44 minutes ago, Zenkman said:

I received a 30-day extension on a Non-O marriage visa last year,

Sorry you most definitely did NOT get any 30 day extension on a 90 day Non-Immigrant type visa. There is no such extension available 

You had to have come in free stamp 60 day visa exempt OR on a real 60 day tourist visa entry to get a 30 day extension, those are the only two things that have 30 day extensions

If you are indeed here on a 90 day Non-O visa (which would show on your entry stamp as Non-O) and you'd be stamped in for 90 days you either get the 60 day visit thai family or you get the year. There's nothing else 

Could you upload your entry stamp from last year and the 30 day extension you got from it?

I would almost bet dollars to durian you got stamped in on a ผ.60-ม.17 category 60 day free stamp entry not a Non-Imm visa for 90 days