I will soon do a one year extension for an OA retirement visa. This past year I did not travel outside of Thailand and never bought an exit/reentry permit. However, this next year I will purchase a multiple exit/reentry permit.

If I travel outside Thailand for a few weeks and return, will this change my 90 day reporting date? Or, do I still follow the 90 day date stamp on the bottom of the TM7?

The 90 days start again when you return.

Yes, the clock is reset. The day you will arrive in Thailand will be day 1 of the clock. You report 90 days afterwrds.