Thank you very much for your help, jochen_vdk!
I did exactly that and received today the answer from TAP that they will reimburse 800 EUR in cash or 1.200 EUR as travel voucher (we were 2 pax in the booking).
Meanwhile, several specialized lawyers confirmed that they would actually hold TAP liable for my losses and that skipping the last leg does NOT make me lose my right to compensation for damages.