QueueClient expects a message that is in UTF-8 format. This is from the documentation:

A message must be in a format that can be included in an XML request with UTF-8 encoding.

The way to serialize your object was causing problems. Using something like this will work:

var queueMessage = System.Text.Json.JsonSerializer.Serialize(item);

because this implementation internally uses Utf8JsonWriter.

Most likely the old library that you ended up using does support Base64 encoding.