For maximum EEPROM readability into the future is it better to write once and leave it, or periodically "refresh"?
1) Use a forward error correction system which offers to recover from multiple missing bits. Distribute the FEC data across different blocks in the EEPROM.

2) Keep at least two whole separate copies to allow re-writing.

2) Periodically re-read and check for any bit errors. Re-write damaged bits. If they don't re-write cleanly, report an error (how? to whom?)

4) Pay huge attention to your board design and part selection; watch out for rotting capacitors etc.

5) You might want to consider "single event upset" (SEU) rates in the processor: while this is normally only a consideration for aerospace, you're running the thing for much longer.

EEPROMS are floating charge devices. They do leak over time, so I think the retention is from the most recent write cycle.