I'm on a non immig family visa. If I apply for extension of stay beyond 90d, will I need proof of 400.000b on a Thai bank account? 

you can apply for a 60 day visit Thai family extension, no financials required, i would wait until closer to the 26th September before applying as you are covered by the amnesty at the moment, a yearly extension will involve financials.

Edited July 24, 20205 yr by steve187

OP, what is your plan? 

If it's simply to apply for 60 day extension to visit family (60 day) you would wait until few days prior to Sept 26.

If your intention is to obtain a 12 month extension based on marriage, then you should have you 400k in the bank or income and apply for extension to visit family and subsequently apply for non o marriage and then extension (marriage)

My plan is to stay until my visa expires in March. I'm arriving in Bangkok Tomorrow and will be locked up in quaranten for 14d. When I get out, I'll get a bank account and transfer the 400k Baht. 

If I do the 60d extension I'll be able to stay 5 months, so that'll be late December. Maybe the boarders are open and flight operating normaly by then. But I don't want to risk anything. Ive been stranded in Europe for 1/2 year...