SPG points certainly are a popular currency to convert to JMB points.

If you want to use your miles to upgrade, you are limited to JMB. Just note that in order to upgrade to business class for miles, you need to be booked in K or higher.

If you go Asia Miles you can earn from your JAL flights as well. And you can redeem JAL flights. But you will as mentioned not be able to upgrade your flights.

Which credit cards to prefer I can't comment on, as I am not US based.