What options do I have for extending my marriage visa in Thailand without the immediate 400k baht deposit?

They pretty much outlined your options;

you get 60 day covid or a 60 day visit thai family to buy you 2 months more time

OR

you pony up the 400K baht and get the year extension 2 months later. :/ That's about all you got

You have two options:

1. Apply for a 60 day extension of stay based on visiting Thai family.

1. Your passport and a copy of:

- Your data ("picture") page

- Your Non-O visa

- Your entry stamp

- Your departure card

2. Your wifePHYSICALLY next to you, and

- The wife ID card and a copy of her ID card

- The housebook, and a copy of the first page

4. Marriage certificate (either the rose border certificate, OR a translated copy from your home country).

5. The application form

6. 2x passport pictures

7. 1,900 THB for the processing fee.

* * *

2. Apply for a 60 day extension of stay based on unable to travel due to COVID-19. The last day the government is selling this is July 29, 2021 (for now).

1. Your passport and:

a. a copy of your data ("picture") page

b. a copy of your most recent visa (if you have one)

c. a copy of your most recent stamp

d. a copy of your departure card

2. A TM30, filed in the immigration office.

3. The forms (each immigration office may have different forms, and ant to use their special forms)

4. A letter from your embassy saying you can not return, OR a print out from your embassy website saying they don’t issue the letters, OR a print out of a letter from the embassy saying they don't issue those letters.

5. 2x passport pictures

6. 1,900 THB.

Different immigration offices have different policies. *SOME* may require:

- The ID card of the property owner where you are staying

- Copies of *ALL* stamped pages in your passport