Volume is not important as long as the liquidity is there. And there is enough for you.
https://forum.mustachianpost.com/t/liquidity-of-european-etfs-when-making-large-trades/6904/3
Swissquote have a fixed price of 9 CHF for buying ETFs at SIX. That’s probably what you see. In the app you can see all costs separately, from what I remember.
You can buy it for USD at SIX or for EUR at XETRA, but for a Swiss resident EUR is much easier to handle and free to transfer.
With a Swiss broker, you pay Swiss stamp duty when you buy foreign securities on ANY exchange. Otherwise this forum would have 40% less messages.
Calculate total costs including currency conversion. And the answer will be - buy at degiro and request a transfer from time to time.