Having travelled on both Ships I would have no concerns re Trollfjord (I travelled on her late October).
She is a bigger ship than Kong Harold with more passengers - but more modern and there is more space in public areas per passenger. (Length TF: 136 metres; KH 122 metres Berths: TF 576; KH 498)
TF is due for a refit and there are a (very) few places where this is visible but remember this is a public transport vessel subject to Norwegian Laws/Regulations and Norway is one of the richest world countries with the best Health Systems. I find the thought that a Norwegian flagged vessel working in Norwegian waters could have ventilation not up to par unrealistic. I think it's more likely to be better than the vast majority of cruise ships flagged in "convenience" countries throughout the world.
I hope this is reassuring - happy to answer any questions.
Fair Winds & Weather