Have you looked at SnugPak?
No, just had a look there. Looks like there is one that is lighter than most I've come across so far though. Is around 500g completely unrealistic?
https://www.ultralightoutdoorgear.co.uk/equipment-c3/sleeping-bags-c21/sleeping-bags-c88/synthetic-bags-t743
@jakemcree started
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Have you looked at SnugPak?