Interesting thoughts. The Blackout becomes very affordable if one reloads. However, the blackout still might not cycle when not suppressed, even when using an extremely short gas piston. The shortest are merely plugs on the end of the bolt carrier.
The problem with the 9x39 is that the AK-9 uses a proprietary black polymer magazine which is not available.
Here are some additional options, that are far less expedient but possible immediately for a Galil. Takeoffs of Ace plastic lower using ar magazines. This permits the use of Wilson Tactical Lancer modified magazines the .338 short straight becomes an option as well.
This also allows the uses of 6.8 magazines, now the .338 spectre which opens the neck of the 6.8 SPC to .338 becomes an option. It is very much the Americanized version of the 9x39.
I expect that the increased mass of the bolt carrier group and spring tension of the AK system that the 9x39 or its Americanized cousin would be far easier to tune than the blackout. After all, it has been done already with the AK-9.
An odd final thought, if you are willing to heavily modify Grendel magazines and trim Carcano brass down to 43mm then .40cal/10mm becomes an option. I do not think that the .400ar is a better option than what you have already considered. It would definitely be more work. But for those who want more than a 9mm, the extra effort might be justified.
EDITED: to explain the extreme deviation from the simplicity of nhgunnut intent.