Transfer just the minimal amount of business data in between Camunda and your third party tasklist application.
For creating tasks, transfer just the taskId and important business data references/ids to your domain objects. As much as possible should be retrieved later, and just when needed (e.g. when displaying task forms to the user) by requesting data from the process engine or by requesting data directly from other systems.
For completing tasks, transfer just the business data which originated from Camunda and was changed by the user. This means, in case you just maintain references, nothing needs to be transferred back. All other business data changed by the user should be directly transfered to the affected systems.
As a consequence, you may have to face that your loosely coupled architecture does not allow to commit all the necessary updates (business data updates and the task completion for Camunda) within one transactional operation. To deal with that, consider to call the task completion for Camunda as last step, to catch any exception arising in any of the called systems and to react to such an exception by re-establishing an open user task within the third party tasklist. The user can revisit that task later and deal with the presented business data situation.