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It's not an easy fix because you can't guarantee what you would be fixing to will hold up. A piece of aluminium chequer board plate would cover the outside, epoxy it on. You could cut a strip of the bottom of the door and use marine ply, the aluminium would cover the join. Epoxy it all together and you have a proper bodge.
The lower quality doors are reasonably cheap to buy, fitting is probably less work than repairing the existing but painting can be more time consuming and there's no guarantee that a new door won't warp, split or leech resin if it's made of pine.
Last time I did this job properly it cost around £40 for a door from Howdens, painting took around 4 hours in total, I was also painting other doors the same colour. I had to reject 3 doors that were badly made or damaged from the supplier.
Because it's quite high up, I'm wondering about whether a larger bottom lintel(?) would work. But then it won't match the other doors.
Otherwise I planned to cut the wood out, reinforced along the back like below, and blend/ fill in. Although now I've drawn it, maybe just an additional strip along the bottom is best?
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