@Merwin ******
You may still apply for a six-month multiple-entry tourist visa prior to departure. This gives you up to nine months. I assume that she also meets the requirements for the 60-day visa exemption. In this case, it is recommended to begin with that option and contact a licensed transport company to inquire about the practical duration for continuing border runs in order to obtain new visa exemptions. At the same time, you could explore the possibility of applying for a DTV from a neighboring country, provided that the relevant eligibility criteria can be met.
Subsequently, you may in the end apply for a Non-O visa within Thailand, supported by an income certification issued by your embassy, provided that you meet the minimum monthly income requirement. Should this not be the case, you will need to submit your application outside of Thailand in order to be eligible to open a Thai bank account.