Advice: Rotten wood trims on external porch

[BillyWillySilly]

Hi there,
I'm stripping our porch which has shiplap on one side. The ends have wooden trims, on of which has rotted (I suspect due to not being properly sealed on the end, which was in touch with the concrete below). The intention is to strip all wood back, seal/silicone etc and repaint. I just wanted advice on whether I need to treat or pack the area where the trims were nailed to the structure of the porch?

 

[foxhole]

Always helps to treat timber, considering replacing will cement board as it doesn't rot.