Wow ! not "ironically" you found some gauls , sounds more like this project was "meant to be "
To get a decent amount of good red rust, getting a hold of some Iron filings (commercially widely available) & oxidising them in distilled water would seem like the most efficient way to do it, it wouldn't take long at all.
After you've read those links especially the first one you'll be a lot better in formed.
You say - "But would using shellac as a binder seal out oxygen and prevent that reaction? Would gum Arabic work better? Is there a third option that would let the iron breathe more?" - No worries there- the shellac will make a hard sealed surface BUT ONLY when it is put on the page & left to dry 24hours or so.Anyway
the the shellac is not added to the very final stages -no need for any concern about that aspect at all. Still though I do strongly suspect that Rembrandt would have used gum arabic because it had been coming down the silk road for millennia where as shellac was hardly used at that time (I don't think?) The reason i recommended you using shellac was because you seemed to be attracted to multi layered washes. this would not be so easy with gum arabic of course because it would be more like using water colour paint.
Do remember though that one of the main purposes of putting in either the shellac or the G/A is to make the ink just the right viscosity to function properly with the pen although I know you want a shiny & glazed finish that is most definitely a Shellac finish .
Mould in ink is a concern ,the addition of borax makes it too neutral a PH for mould to survive, BUT
why havn't I heard of this being used in iron gaul before ? certainly it would prevent the initial highly acidic chemical reaction that is essential & integral to the iron gaul ink making process but would adding it later after it is made kill the ink? I don't know. However I'd be careful here as if all the major manufacturers sell acid
iron gaul ink to this day(& they DO!) then it won't be any deficiency in their knowledge of chemistry or experience ! tread carefully here.
Here is another Really good link containing other links within it -fountainpennetwork.com/forum/topic/249871-how-to-make-iron-gall-ink/
There are some discussions on paper there too, I'm afraid I'm at the end of my limited iron gaul knowledge.Can't wait to see your resulting drawings though !