I think you need to ask to ask the Oracle Tod Danielss but I remember reading a few years back that under the five year elite visa, if you border-bounced towards the expiry date, you'd get stamped in for a year, effectively giving you almost six years. I don't know if this concession still exists because it was a few years ago, and it may not apply to the new scheme.
has said, when you're on a 12-month Retirement extension you just have to obtain a re-entry permit before leaving Thailand (you can buy these at the airport), if you're using proof of pension you need to get your pension payment certified by way of affidavit at your embassy in Thailand. This must show minimum of equivalent
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baht per month. Generally you will need to show a 12-month lease for where you are living (it doesn't matter whether you have just started the lease, or are halfway through it, as long as it is 12 months). I say "generally" as each immigration office make their own rules on this, so best to check there first.
I don't know what you mean by "planned trips" but are you aware he can apply for the 30-day extension the day after he arrives, giving him a clear 59 days stay?
Start the 65k/month process during the first year you're in Thailand. Get the second year free by way of border bounce. By the third year you'll already have the required 12 months so you can either extend the OA in country, or alternatively leave, come back on an exempt entry, get the non-O still using the 65k process, but DO NOT miss a single month or you're up shit creek without a paddle.