Post Millennial is anyone born after the millennial generation(including people who are/can be part of Gen Alpha).If you are trying to describe post millennial as in Gen Z people with millennial overlap,that's wrong there's no millennial overlap at all,even with the oldest of Gen Zers. It's a lot more simple to just say Gen Z, as opposed to "post millennial for the 1st half of Gen Z and igen for the second half of Gen Z". We can separate the first half of Gen Z from the second of Gen Z like this,using my 1997-2014 definition.
1997-2005- 1st half for Gen Z(1997-2002 would be fully early Gen Z with 2003 and 2004 being cusp between early Gen Z and core Gen Z with 2005 being fully core Gen Z)
2006-2014-second half of Gen Z (people born from 2006-2007 would be fully core Gen Z with people born from 2008-2009 being cusp between core Gen Z an late Gen Z.2010 would be fully late Gen Z with 2011-2018 would be Gen Z/Gen Alpha cusp)