This is an exterior door into a detached garage. We know we have water intrusion here, and are dealing with that issue separately. We're having the house painted right now, though, and the painters brought the rot to our attention during their prep. (We hadn't noticed how bad it was since this isn't a part of the door we look at often.)

Can/should we try to fix this, or should this door just be replaced? It is a 15-year-old prehung door, most likely an inexpensive big box option of some sort.

We don't care much about the aesthetics, but don't want to risk rot spreading from the door into the garage framing. There is a chance of this recurring until we are able to get the drainage adequately addressed here, though--this is the low point of the property--so I don't love the idea of putting in a new door before then. If there's a way to passably repair this one to get a few more years out of it, that would be the first choice. If it matters, we are in California in a climate that is dry most of the year but gets periodic winter rain.

If we repair it, what are the right products to treat/fill this? If we replace it, should we buy a new prehung door and replace the entire thing, or is there a way to buy just the door itself? (I can't figure out what brand this is, though it looks very similar to many of the basic steel options sold at HD.) Thanks!