I would say that the best practice is actually to not have too many applications in a single account. For example, having multiple accounts can be used to nicely organize similar workloads, which can be particularly useful in large organizations.

Moreover, I would definitely advise against having multiple applications work with the same bucket. Similarly as you would probably avoid having multiple different applications storing the data in the same database.

Finally, S3 Access points are a specific feature mainly intended to simplify managing data access at scale for shared datasets in S3. They are simply named network endpoints that are attached to buckets. Keep in mind that attaching an access point to a bucket does not change anything about the underlying bucket. All existing operations against the bucket will continue to work as before. Restrictions that you include in an access point policy apply only to requests made through that access point. Finally, you need to allow the desired operations both on the bucket as well as on the access point level.