Door threshold help

Hello!

I recently purchased a home that has a sagging exterior door threshold (I think it's a threshold?) when stepped on. I believe this sag caused rotting of the door trim, which is one of the first things I had replaced.

Original issue: rear entry door trim rot


My guess is that there was originally caulking between the trim and threshold, but the caulking was compromised because the entire threshold would move when stepped on.

I had the threshold sagging repaired but I don't think the repair went far enough. The repair person used a 1x3 board (not treated) board underneath the threshold to prop it up. It helped, but there was still sagging. In the video you will see the leftover nails where I pulled

https://youtu.be/lsQ-3HyZ-yQ

What is the right way to repair the sagging ? Two thougsh:

1.) Place some shims under the threshold ? If so, should they be treated, plastic ?

2.) Take out the sill and replace it with concrete ?

I appreciate any advice!

Thank you,
John

This is a picture of me pressing down on the threshold, along with what I am calling a sill and threshold


This is a picture of the uncompressed threshold.


A picture of the sill (under the threshold)


Overview picture


Overview picture 2

 

Plastic shims. Treated wood eats aluminum. Then place a 1x4 under the door sill (pvc is best) and fasten it to the wood under your door. This will help support the front of the sill. (A threshold is the part that is directly under the door when its closed.)

 

I appreciate the advice! Thank you!