What's the best way to update several (200k)rows in a database?
I would suggest that the schema is poorly designed if you have 205K rows with the identical uuid.

Furthermore, if you need all of them to have the same ended_at value, then there should be some other table that has, for example, one row with uuid and ended_at.  The JOIN to get the data during a SELECT will be more than compensated for by having removed ended_at from the big table.

Perhaps I am being overly critical.  If so, please provide some more insight into what the columns mean, and where the uuids come from, etc.

The best way to update "many" rows in a "huge" table is to walk through the table by the PRIMARY KEY, and update a thousand rows at a time.  More details on chunking:  http://mysql.rjweb.org/doc.php/deletebig#deleting_in_chunks