It's been my experience that most customers leave $SPLUNK_DB commented out in splunk-launch.conf and let the index locations be specified by index.conf files.
When you change the default location you are also changing the expected location of existing indexes and/or summaries. Therefore, it's important to move the existing files to the new location. The sequence is:
Edit indexes .conf
Run btool check to check the config files
Stop the indexer
Move files from the old location to the new one
Start the indexer
--- If this reply helps you, an upvote would be appreciated.