In Postgresql 12 how to I make the primary server a slave after a deliberate failover
Sounds about right, but I would keep the configuration identical on both servers, so that you don't have to change anything:

recovery settings like restore_command are ignored on the primary

you can set archive_mode and archive_command on the standby, since it doesn't archive WAL anyway (unless you set archive_mode = always)


Before you remove the data directory and re-initialize the standby with pg_basebackup, you could try to run pg_rewind, which does the same, but faster.