@Mas-E - Are you saying you also want to assign out subtasks of subtasks? I think it’s generally agreed that going beyond one layer of subtasks gets quite messy unless you’re just using them as a punch list instead of assignable items.
There are some other workarounds (e.g., multi-home all subtasks to another project to enable bulk-editing another layer down), but short of using the API and a 3rd party integrator, I don’t know how you would accomplish your request with more than one layer without lots of complications…
You could consider giving this post a vote to enable the bulk-edit option to work for your use case: