During February and March 2020 I was responsible for the FE migration of a multi store and heavily customized multi theme Magento Commerce store.
This was an extensive job given the many customizations and overrides within the various themes. Luckily my years of experience with Magento upgrades helped a lot here (being able to extract a good 'diff' of all files, being able to assess what must and what must not be updated, understanding implications of changes and documenting this all in Confluence so creating a good reference for future upgrades).
The result: a smooth upgrade and launch without any production issues. Mission accomplished!