The documentation perhaps does not explain the behavior properly. but anyway… A write operation in any database should not block parallel read operations. Read-last-committed as isolation is what one expect but not blocking of read operations until the end of the write transaction.
It seems we can’t make you happy with BaseX, Andreas. Sorry for that. In order to allow concurrent reads during updates and at the same time preserve all ACID guarantees and other guarantees that we can give to our users, we might switch to an MVCC architecture one day in future. It might get reality if we find enough sponsors out there.
The current architecture works fine in practice, though; it is applied in commercial use cases in which millions of datasets are stored and thousands of concurrent updates and reads are performed in realtime by hundreds of users. However, building such applications requires some knowledge and experience with BaseX. One crucial issue is, for example, that your update operations will be fast.