If Elfa drywall and plaster anchors can hold shelving, they should be able to hold a TV. I don't think it is necessary to open the wall. Drywall supported by a pair of steel studs should be sufficient.
Use four of these anchors, or their equivalent, through the drywall and through the edges of the steel studs to clamp a suitable piece of plywood to the studs and drywall. The 5th drywall anchor (they come in pkg of 5) could be used in the top middle even though there is no stud there.
The plywood would be 26" wide by whatever height is required. The TV bracket would be screwed into the plywood.
The instructions for the Elfa anchors specify 10 mm (or 3/8") dia holes in the drywall (and steel studs), but the holes in the plywood must be 5 mm dia (or 3/16"), just large enough to allow the shaft of each screw to pass through.
The exact procedure would be to mark the centers of the two studs. Hold the piece of plywood onto the wall level and so it overlaps the centers of the two studs by an inch on each side. Drill 5 mm holes in the five locations through the plywood, through the drywall, and through the edges of the studs. Remove the plywood and drill the holes in the drywall and studs to 10 mm. Use the 10 mm drill to countersink the 5 mm holes in the plywood for the flathead machine screws of the anchors. Note that these anchors are limited to total wall thickness of 1" including the plywood so countersink accordingly. Any anchor that clamps on the back vtge drywall will have a limit.
Insert the anchors in the holes and drive them so the outer edge is flush to the drywall. Fasten the plywood to the wall with the five anchors. Use an electric driver to turn the screws because these are fine thread machine screws which take a lot of turns.