Domestic transfer airport help

I am heading in from abroad to Japan and heading directly on with domestic flight as train is too time consuming.

I got Narita or Kansai to choose from and I know there are not so many domestic flights there as from "nearby" Tokyo-Haneda or Osaki-Itami ariports.

Say. Which would be the safest bet to go with considering transfer time after landing and safely being at the gate meaning I have no risk in missing my flight?

I expect possible delay of arrival and traffic jams between the airports... Not sure which would be fastest for check out (immigration, customs) and check in as I have never travelled from those.

They are mostly the same. Bus transfer between Kansai and Itami is currently free of charge for connecting passengers.