Self commenting on update progress, in case it helps anyone else.

I've increased max PHP time and maxed out memory allowed. Profiles migrated successfully. I then started on Paid Memberships levels, Members info... but ran into a problem.

Query:
INSERT INTO `sys_acl_levels` SET `Name` = ?, `Icon` = ?, `Description` = ?, `Active` = 'yes', `Order` = ?

It turns out I have a membership level that has a Name field that collides with an existing record in a default install and database table has a Unique constraint on the index for that field. For now, I changed the index to full text and it's imported OK. Just not sure how this will impact down the line yet.