This happened to us on Eurodam last year (2021). There was a smell of sewage in our bathroom, which occurred mostly at night. Eventually it disappeared. They almost took our bathroom apart and rebuilt it several times, but apart from the smell, we had very little inconvenience (work was done during days ashore).
On the last evening there was an envelope with letter from Hotel Manager, apologising for the inconvenience and advising us that a not unsubstantial amount of OBC had been credited to our account. I went to Guest Services and they confirmed that I could 'cash in' the OBC as it was our last night.
So perhaps a firm, but polite, note addressed to the Hotel Manager may bring more suitable compensation for your inconvenience.