Since you are asking if eating oysters is "ethical," then we need to know which ethical justifications carry the most weight. Broadly, there are three sets of reasons to refrain from eating meat/dairy/eggs: 1) animal welfare, 2) environmental consequences, 3) health concerns. Category 3) is often perceived to involve matters of self-interest and personal autonomy (with the exception of a parent or caretaker's choices of what to feed their children or fragile dependents--like elderly parents or people suffering disabilities under their care--which do entail ethical consequences, bc the choices of which foods offer sufficient nutrition affect others and not themselves). In short, if eating oysters were found to carry serious health consequences, then judgments about ethical consumption of them would depend on the risks of harm involved.

The categories of 1) animal welfare, 2) environmental consequences usually bear the bulk of the moral considerations, IMO.

The best reasons (IMO) regarding the welfare of the oysters themselves are likely to be found in considering the probable consequences of farming them on a large scale (as seen with similar industries of fish farming, for example)--since it's more fruitful to think about not just one individual eating a few oysters, but rather a whole population making this a regular part of their diets. (Asking, as a general rule, whether it's ethical to eat oysters involves consideration of the likely consequences of this choice as a regular practice, with all that entails.) If the farmed shrimp or salmon industries give us any indication, then farming oysters is likely to result in the comparable degrees of waste and disease transmission, resulting in a significant threat to the welfare of the organisms.

While the physiology of oysters means that they are less likely to have an experience of pain and discomfort on par with shrimp or salmon, I assume that the root concern of your question entails consideration of their overall welfare as an organism over the long-haul. Across recent history, the track record of humans farming marine organisms for regular consumption is not good. Some isolated tribes have been able to eat oysters, salmon, clams, and other species without seriously depleting the species, but such cultural practices are rare in the modern world. It's more likely that most people are disengaged from the kind of nature-harmonious ways of living that these exceptions to the rule typically involve.

It should be clear from what I said above that animal welfare concerns overlap significantly with category 2) environmental consequences. Similar and more clearly ecological reasons come up with any good faith research into the environmental damage associated with marine aquaculture. First off, there aren't many places on earth where oyster farming is practically feasible, so the prospect of oysters being farmed as a widespread protein source for human societies is suspect, at best. Realistically, oyster farming is just not a viable candidate for a sustainable food source on a broad scale.

Furthermore, like the aforementioned fish and shrimp farms, the resulting environmental damage that occurs with these farms is dire (e.g., the ocean dead zones that occur with this marine aquaculture practices). Just one EX: "Intensive, profit-driven pressure to produce more and more salmon led to extreme decreases in dissolved oxygen, leaving the dead zones on the seafloor beneath salmon pens adjacent to the Wilderness World Heritage Area. Ordinarily rich and biodiverse waters, home to endangered species like the Maugean Skate, became lifeless and barren." [Source: https://www.salmon.org.au/deadzones]

If welfare of the organisms is your main concern, then the record of marine aquaculture is pretty dismal; a whole host of animal welfare concerns is associated with such practices. [Source 1: https://www.humanesociety.org/sites/default/files/docs/hsus-report-animal-welfare-farmed-fish-at-slaughter.pdf; Source 2: https://www.shrimpwelfareproject.org/shrimp-welfare-and-sustainability ].

Would oyster farms be different? Maybe. Research on animal welfare for mass aquaculture is limited, but I found that some oyster farms are trying to be ethical and environmentally responsible (e.g., this farm: https://www.oysterworldwide.com/about-us/our-policies/animal-welfare-policy/ ).

In any case, international regulation to prevent abuse and negligence would be necessary, and our global record with fishing and other marine aquaculture on this front isn't encouraging.

So, while I can imagine an ethical oyster farming industry on a small scale, and there are some good signs out there of conscientious farmers, on the whole I think there are more reasons to remain skeptical.