The EU hasn't been very helpful on this in the past few months when even fairly reputable airlines like Lufthansa didn't pay back on time (as specified by the law). It's a massive liquidity problem for airlines, and when considering the big picture, the EU executive may prefer to go easy on airlines at the cost of customers (who're loosing non-existential ...
If you paid (part) by card, file a chargeback through your bank for that amount. Attach screenshots of any written correspondence and specify at what date and time you've called Norwegian's customer service. Explain you've been promised a refund (say when) but still haven't got it.
My strong guess is you'll get back the amount paid by card.
I think you should wait. Maybe you get some hits on national newspapers.
As you know, COVID heavily affected air industry, and so they lack of liquidity. For this reason countries tend also to tolerate delays (but usually they requires refund in case of bailout credits): better delay then a bankruptcy.
In any case, in https://europa.eu/youreurope/citizens/...
As far as I am aware, I am eligible for a refund of my entire ticket due to this flight cancellation
However, could I also opt to only fly the return journey, and get (roughly) half of my ticket value back?
That's unlikely to happen since LH would have to reprice the ticket which is complicated and will NOT result in a 50% refund. I'm guessing ...