Creating a globally consistent, strictly increasing, small integer number mechanism is very hard to do. A reason why ZooKeeper, atomix, etcd, consul, etc, are so widely used is because they do the hard work of implementing value consensus in distributed systems. For my hobby project these tools are overkill, but I still need the mechanism.
To restate the requirement, it is a mechanism enabling multiple processes (on a distributed set of machines) to add tasks to workspaces. But is that really the requirement? Isn't it a premature optimization to assume the need for multiple processes? Perhaps one process can handle all the task creation requests for a single workspace?  If I have 100 workspaces I can choose to enable task creation by distributing any request to any process, but I could also choose to distribute task creation to the workspace specific process. Choosing the second option vastly reduces the problem to enabling a single process to sequence the task ids. All I have to do now is direct the request to the right process and this is easily done at Slack with a different URL (option A) during application installation, or at an application layer gateway (option B).
So I am going with one process per workspace  with option A.
 An assumption is that one process is sufficient to manage the demand of a single workspace.
 Note that this does not preclude more than one workspace per process.