Your correct about "O" rings (good thing you did not design ET3)
O-rings in jet engines occasionally fail and have lead to explosions or fires resulting in crashed aircraft. This is one reason they will not likely be used for ET3 seals. Have you ever considered how the old style "picture tube" type of TV could maintain 10ee-7 Torr vacuum (a thousand times higher quality vacuum than needed for ET3) for 30-40 years with only rare failure?