Sub-tasks are imported as checklists by default, but you can change this behavior and import Sub-tasks as regular cards. In this case, these cards will have connections with the parent cards. You can also move Sub-Task issue type to a separate board in the same way as other issue types.
On «Board structure configuration» stage in selector «import Sub-tasks as:» select the «cards» item
After that, a new Sub-task issue type will appear on the first board
Setting up the configuration of the boards. Click "Next"
Sub-tasks in Jira inside some issue:
The result of importing to FlowFast: