Bank certification with the proof you have 400k on deposit. Usually a single paper signed statement and that was enough before; but since my bank book had a 3 months dark window of not being updated they requested a list of complete transactions for that time as well.
You don't need 400k/800k baht to get a non O Visa, some embassy will maybe ask you to provide a bank statement with a balance of the equivalent of 6000 Euro or more (not required in Ho Chi Minh Thai consulate for now). You need to have 400k or 800k in a Thai bank if you do an in country extension of an existing non O Visa tho
It might be cheaper to apply for an extension and do a TM8 to get a multi re-entry permit for 3800 baht. A 1 year multi entry visa usually costs USD 200
Amnesty is until July 31st, however if I was you I would extend now, for sure imm offices would be super overcrowded after that... extensions requirements are still the same, that's 400k baht financial proof, pictures of you and spouse together, tm30 ect ect