The difference between the 6 months︀ extension and the 12 months extension is the price - and the fact that there︁ is no work permit required for applying for the 6 months visa extension - legally︂ it might be necessary, but it's not being forced.
With just the 30 day tourist︃ / business visa it's difficult, but not impossible to open an account, depends on the︄ branch. Having a stash of money and a rental contract, and meeting the branch manager︅ might make it possible, the success rate wouldn't be higher than 20% though and one︆ needs to go from one branch to another to try again - in the heat.︇
I actually opened my account at the biggest bank in Cambodia many years ago with︈ just a tourist visa and a
guesthouse address - I still treasure my 4 digit︉ long account number, with lots of zeros in the front - but this would be︊ really difficult now.
With the 6 month extension and a rental contract the success rate︋ is 80% (a Maybank branch kept pushing for the work permit or a receipt of︌ the application for it though, other bank told me that I have to marry a︍ local first).
With the 12 months extension and a work permit in hand, the success︎ rate should be 99.9%.
Oh, don't forget to check your status at the bank -️ tell them you are resident (with your visa you officially are, especially with the 12 months extension, and the tax on interest changes from two digits to one. Half of my banks forgot that and I had to remind them myself.