Yeah going with US ISIN’s.
Estate Tax has been covered lots of times here, its not really an issue, the ‘limit’ is not 60’000 but 11Mio. Also there is no documented case we are aware of where a Swiss citizen was taxed on his estate, the way you might be afraid of. I guess you have to watch out for changes to the treaty while accumulating but thats about it.
Whats your rationale with the butterfly portfolio, as opposed to a more conventional stock/bond split?