Hi,
The current* LTS is 6.7. There are no other LTSs between it and 7.5, so you can go there without any intervening upgrades. That said, there are no extra reporting capabilities in 7.5 that were not in 6.7.
Regarding MySQL, support is not deprecated in 7.9, it is removed. However, we’ve provided a free utility to move your data from MySQL to a supported database. See this announcement.
Ann
* SonarQube 7.9 LTS is expected early next week