I did not know about the manual rework of the transcoded Z80 code, thanks for the precision.
Dynamic sprite multiplexing is feasible, Silkworm does it constantly with the missile/bullet sprites and it works great even if sometimes one or two sprites cannot display because the game goes over the sprite-count-per-scanline limit.
Why do you mention 6 sprites as the horizontal limit? As far as I can see there is no horizontal scrolling so no sprites are "eaten" by the extra scroll DMA fetch, is the game overscan or is it because you are using the last two sprites for the player ship?
Maybe it would be worth using the Blitter for the player ship to gain two extra sprites for the ennemies.
This said, as you hinted, even 8 sprites per scanline are way under what Galaga displays so this would still be a challenge indeed. Maybe a combination of Blitter and hardware sprites would have to be used but this would complicate even further any "object multiplexer". For such a simple game, Galaga is indeed tricky to adapt.
In any case, good job! 👍