I don't think I'd ever use a convert all button. I'd imagine this is a likely source of "I just lost all my gear!" forum posts more than anything else.
As you say, there is already a way to scrap max. Maybe it could be in the number button? 1,10,50,max? Sure, that could be there. Personally I always keep like 50 of anything, more for salvage pieces, so again, not a feature I'd likely use myself, but not against it. I don't think it would cause issues for some like a scrap all button might.
What ship gear? Upgrade droids? I'd rather never have to think about them again. Nothing wrong with having a massive pile of resources. People asked for a way to use gear, and we got the relic scrapping system. No need to add more items to the list of stuff to worry about running out of.