I engage hams and glutes with the oldschool pulling up of the offload pedal. It’s all integral with the modern era term and practice of spinning circles. Every rider has their own tweak to this but if you will do hill rides with the pulling on the upstroke pedal you will find glutes and hams with plenty of action. As for your backpain, you may be pulling on the bar a bit more than realized. Be patient, we aren’t young anymore. And do check your seat height to have a full stroke on the pedal (but not so high to lockout the knee or cause bobbing on the saddle in higher cadence situation).