What is the issue with receiving tickets as PDF? It is the most flexible yet digital option. PDFs work regardless of the medium. You can show it in the phone or you can always add them to DB Navigator App anyways or if you would like to be old school, just print them.
NFC is limited access for phones. You need to pay Google and Apple tax. Android does allow independent communication but it is not widespread and you'll lose a big chunk of Apple users. You basically tie yourself to a platform that way.
You can scan and add your QRs (or more correctly Aztec barcodes) in DB Navigator app already. If you bought it via your own account (instead of your company buying it), you don't even need to do it. The tickets automatically appear.
DB Navigator is one of the best transport apps and already implements some caching. However you're ultimately tied to cell network or WiFi in train for certain othet apps and the quality of implementation. PDFs don't expire.
