If you don’t have any recent excessive travel history in and out of Thailand you can leave and re-enter on another visa exemption. There’s no official limit or regulations regarding this and everything is in the discretion of the IO you’re standing in front of. If they mean that you aren’t a genuine tourist but use tourist solutions for long term stay in Thailand you risk to get pulled aside and told to get a proper visa according to your travel pattern and/or denied entry.
As Miller states you can apply for a 30 days extension at your local immigration first at 1900 baht.
Regular 90 days Non O visa don’t have mandatory health insurance requirements as stated by several members and it’s the best way to retire in Thailand. If you enter on a one year Non O-A visa you’ll maintain the mandatory Thai health insurance requirements also for your future extensions of stay.
Apply for a 90 days Non O before you leave. Set up a bank account with the required 800K baht within a month and apply for your first year extension of stay at the immigration within two months.
I understand, this it’s no official site and there’s in fact nothing such as a 7 days extension, but they will probably give you a 7 days extension denied stamp and know what you’re looking for at the immigration.
Someone might call this an extension on a VOA as it’s not. If you try to apply for two extensions on a tourist entry you’ll also get an extension denied stamp and 7 days to leave.
I don’t have a complete overview but I’ve not seen feedback of someone denied extension denied stamp and 7 days to leave, so you’ll probably be fine.