Safari file save location
I can think of three possibilities:


One option that will definitely do this is to use Default Folder X1. It's not free, but you can use it for free for 30 days. Depending on the version you download, it's compatible with versions of macOS from Snow Leopard to High Sierra.
Default Folder X does a lot more than just setting the default "Save" location of individual applications.
As for a native macOS solution, I seem to recall a bit of a trick users could do that involves creating a smart folder and then adding it to the Sidebar. The idea was that you then navigated to that folder (via the Sidebar) when you next uploaded a file via Safari. Safari would then remember that location when saving files in future, regardless of where you subsequently uploaded files from. However, this is an old trick from about ten years ago and I couldn't vouch for it's validity now.
One possible workaround for you, assuming you're wanting to use the Downloads folder as the default Save location, is to just remember to use the optioncommandL shortcut when the Save As dialog window pops up. This shortcut takes you straight to the Downloads folder within the dialog box.


1. I have no affiliation whatsoever with Default Folder X.