In case it helps anyone I found a circular from the Swiss tax authorities listing Double Tax Agreements by country and how they relate to the source tax charged in Switzerland
Recap: source tax is charged in Switzerland when you have left the country and withdraw 2 or 3 pillar. For countries where the document says “Prestations en capital: Retrocession possible? Oui” this means that the swiss source tax can be reclaimed under DTA (“retrocession”). This is actually bad news because it implies that the lump sum is taxed in the destination country
Example UK
«Prestations en capital - Retrocession possible? – non » : Means that the source tax charged in Switzerland can’t be reclaimed. This is “good news” because per the DTA the capital payment is not taxed in UK (see prev. post) so you only pay the reduced Swiss rate
Example Spain
«Prestations en capital - Retrocession possible? - oui » : The source tax charged in Switzerland can be reclaimed. This implies the capital payment is taxable in Spain which is also confirmed in the DTA. What is not clear is at which tax rate…