Also a "Certificate of Residence" from imm. you use, helps a lot for opening an account with the non O visa. The only thing I read that some imm. offices ( E.g. Bangkok) issues this only after a first 90 days report. So then you cannot get one there. Just ask the imm you gonna use for y'r year extensions. And for 2 separate bank accounts, with both the app installed, you could share the login data/pin.
What I read is that SOME imm. offices accept a total of 1,600,000฿ for 2 persons. But I think it does not have an advantage. This also if one should die, in Thailand, they can block the account untill it's clear who are the heirs.(e.g. family, married, a will).
Okay, existing users can still use the internet site with a browser but y'r right, maybe more will follow and stop using this. I still use the sites sometimes e.g. to make a print. With the apps (don't know Krungsri), making a screenprint is blocked. Also there are a bit more options on the browser sites, e.g. I can see the balance in my Fixed deposit account, which I cannot add in the app. I have Bangkok bank and Kasikorn bank. Hopefuly they continue a while.
No, not needed but maybe a tip. Check y'r airline for refund options. I bought a return flight from EVA Air and when I decided to stay in Thailand, I got a refund of 150€ !! on my 'return home flight', so this made my 'one way' to Thailand 150€ cheaper then when I had booked a one way ticket in the first place.