Firstly the time variance we're talking about is small and will only matter when milliseconds matter, secondly I addressed this - for scientific measures and scenarios where milliseconds matter a different time unit should be used.

If you want to prioritise ease of moving between the two systems, I'd suggest a normalised version of the new second. If you want to prioritise science, then something that makes a nice round number when giving the speed of light would be nice.

Either way the symbol for writing it should be different.