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Window sills material
(lemmy.ca)
3/4” of fir will be plenty strong enough that it won’t break . I have a stool made of 3/4” cedar and that’s close enough. I think yes plant pots could leave small marks though that’s true. Really you can’t go wrong with red oak. It’s strong and cheap and you can even stain it. I say Doug fir will work but you might as well go for red oak. Will last forever.
I made my computer desk from pine and you can see letters on it from where I’ve written. Yaa go for hardwood