A further update:
I received the notification letter from the Gemeinde earlier this week, and have now collected my permit... a 5yr EU/EFTA B !!!
Its "Ausgestellt durch" date is early December, coincidentally(?) the same day as my employment contract's start date.
Whereas its "Einreisedatum" is the date I arrived and signed my rental contract, and subsequently registered my residence from -- back in September.
Both are before 31.12.2020 which is all that matters to me
Now time for a good few Feldschlösschens
Thanks again for everyones' advice!