I don't think foreign mortgage interest can be offset against Swiss income tax, only Swiss mortgage interest payments. If you do move to full declaration, bear in mind that the value of your foreign home will be used to calculate the band of your wealth tax as Belgianmum above highlighted (even if the house is exempt), and all your foreign assets and foreign income need to be declared to the Swiss tax authorities and are subject to wealth tax and income tax. So you may find yourself worse off than before, potentially. Source tax is clean and easy.

I also think that if you earn less than CHF 120K a year and do not own a house in Switzerland, as a B permit holder you may not be able to make a full tax declaration, rather than being subject to source tax or completing the simplified declaration that you linked.