@jmorris FYI: If you boil in alum solution to dissolve the taps as I have, make sure to submerge the whole part — I ended up with a permanent line at the interface of the boiling water that I didn't succeed in polishing out when I dissolved one tap out of a piece of 6160.
@jmorris Here's one reference. Can't remember where I first learned the trick though...
https://bbs.homeshopmachinist.net/forum/general/1927244-broken-tap-removal-with-alum
It's potentially an all-day process.
Also, besides thread forming taps, have you tried spiral flute taps? I've had good luck with them, particularly ones with polished flutes expressly made for aluminum. Mine are from YG-1.