As was stated, the Tender can't safely operate at those chamber pressures.
In a Tender, the 6.5 JDJ gets the nod.
For the Encore, 6.5 Creedmoor, .260 or 6.5x47 Lapua in that order. The 6.5-284 is a heckuva long range cartridge but barrel life is short in comparison. The others mentioned are lighter on recoil, better barrel life and easily shot to 1000 yards.
Another cartridge to consider is the 6.5 Grendel. It, along with the .264 LBC are killer mid range cartridges.