Here is a discussion about the conversion from MyISAM to InnoDB; it answers several of your questions, plus some you have not asked yet:
http://mysql.rjweb.org/doc.php/myisam2innodb
There is no fast way to ENABLE KEYS. It must, by definition, rebuild the indexes. However, there are two ways it might do it. For such large tables you want to see "Repair by sorting", not "Repair by keycache", when you look at SHOW PROCESSLIST. If it says the latter, kill it and figure out what tunable to change.
The InnoDB equivalent will be 2x-3x bigger than your MyISAM data+index today.
Six PARTITIONs? That seems like a small number. Would you care to share SHOW CREATE TABLE and explain why you chose 6?
> but read from multi-threads
If you don't have multiple access to disk (eg RAID striping), there is not much advantage there. Let's see some of your SELECTs.