I enthusiastically recommend Iceland Procruises. They use a very small ship (around 200 passengers) with a variety of cabins at different luxury levels, so something for everyone, and the price can be pretty reasonable. We did a circumnaviagation of Iceland with them that was truly amazing. On the second part of the itinerary, a Viking ship was following us and it seemed like a behomoth by comparison! At one port, our ship could dock while theirs had to tender -- what a difference that made.
You could also check out Celestyal, which now has two ships just a bit under 1300 passenges and mostly cruises the Aegean/Greek Islands. We sailed with them a decade ago on a package tour, when the line was called Louis, and it was fine -- not luxurious at all, but great Greek food.
Small cruise ships have become harder and harder to find, without venturing into luxury yachting. Covid was hard on the cruise industry and some of the operators with the smaller vessels (likely leftover from the days when less than 1300 passengers was common) are no more, such as Pullmantur and Cruise & Maritime, which I always meant to try but missed. Others have shed the older, smaller ships, such as Fred Olson's Boudicca and Black Watch (about 530 passengers) that both went to scrap, and Holland America's 900 passenger Prindsendam, which we were lucky enough to sail before her sale to a German cruise company, Phoenix Reisen.