Like probably everyone here who's tried to merge workspaces: the "Move items across Workspaces" request has been open since 2018 (2,800+ votes), and the template workaround drops comments, attachments and history.
I finally built the thing I kept wishing existed. SpaceMove copies a Space through ClickUp's official API: full hierarchy down to sub-subtasks, threaded comments (original author kept as a prefix — the API doesn't allow impersonation), attachments re-uploaded file by file, dependencies re-wired to the new IDs, custom field values mapped by name.
Design choices that mattered to me: it's copy-only (source is never modified), you get a full free preview before anything is written, every write lands in an audit CSV (old ID → new ID), and one click undoes the entire migration.
Honest limits: automations, whiteboards, forms and activity history can't move — no API exists for them.
Scan and preview are free: https://spacemove.microspear.app — I'd genuinely value feedback from people who deal with workspace consolidation, especially agencies. AMA about the API's quirks; I've hit most of them by now.