Merge multisite subsites

You can export, then import. Check how it is, then delete original.
You can try it on a sandbox if you don’t have localhost setup: https://wpsandbox.net/

When we import the Theme Unit Test data, it works great. But since we added blocks to the data, some of the imported content retains the links to the original URL instead of changing to the new one. This is using the standard WordPress Export/Import.
You can try one of the many plugins for migration that might work better.

Thanks for your reply @joyously.
For the link change maybe a search replace in the db is ok, but the media problem seems trick: how to reassign correct images ids in the shortcode galleries to the newly imported media content ?

Yes, that is a trick, which you should test in a sandbox to see which tools address it. The galleries in the Theme Unit Test data don’t specify IDs in the shortcodes, so it works fine (gallery defaults to all attached images).

Yes, the problem is exactly what you pointed out, the ids inside the shortcode : bad idea for migration… ??
Thank you!