This was also my plan, because I didn't see any way to apply for the correct non-O visa in advance of travel, but members of this group set me straight! You can get the non-O online, but it's hidden/unlabeled on the Thai e-visa site,
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Under Purpose of Visit, Visa Type, choose "Non-Immigrant Visa". Then under Purpose of Visit, choose "Retirement (pensioner aged 50 or above with state pension who wishes to stay in Thailand for no longer than 90 days)". THAT's the non-O visa. Ignore the confusing "state pension" and "no longer than 90 days" verbiage - that's the non-O visa you need, then you can apply for extension after you arrive and open a bank account.