You have neither specified which restic version you’re using nor which commands are run when the problem occurs. Without that information it’s not possible to give any proper advice.
To get an idea of the expect memory requirements for the repository please run restic stats --mode debug.
A 6-7GB index is excessively large for a 1.2TB repository. That roughly means that there are 150 million blobs in the repository (assuming repo format version 2). Has the repository every been pruned?
The rule of thumb is that 1GB RAM is necessary for every 7 millions blobs. So no matter which options you try to specify, at that repository size the memory usage cannot go much below 22GB. (That is using the latest restic version, older version can use much more memory).