Google's John Mueller said merging one site into another is not the same thing as a site moving from one domain to another. You should not use the change of address tool when doing a site merge, it should only be used when moving a site fully to a new domain.
Here are those relevant tweets:
No. Merging isn't moving.
— 🍌 John 🍌 (@JohnMu) October 22, 2019
So do not use the change of address tool in Google Search Console for site merges. John said "No. Merging isn't moving."
Forum discussion at Twitter.
