Adjusting the my.cnf file for DB servers after an instance resize
The most important change would be
innodb_buffer_pool_size=6G --&gt; 12G

The number of buffer_pool_instances should be 1 per GB, not one per core.  So, 12 in your new machine.  However, there is a limit of 16.  As for why...

I suspect it makes very little difference -- perhaps single-digit-percentage when the server is extremely busy.  (If a server is that busy, it is about to keel over; you have worse problems.)

Essentially nothing else requires changing.
After you have been running normally for at least a day, grab SHOW GLOBAL STATUS and SHOW VARIABLES; then get an analysis here:  http://mysql.rjweb.org/doc.php/mysql_analysis#tuning