Okay I found that targeting the IAS for OPT Cruise in climb give a faster rate of climb than targeting the IAS for Current Cruise up to 20,000. The higher speed climb then became more effective to 30,000. This under 98% MGTOW, 42C OAT, adjusting throttle for 102%rpm, Cruise Flap setting, attitude hold on for the duration. Once up there I did the fencing maneuver. It was slow but doable. Flaps Auto is really a boat anchor up there.
I had previously found that targeting the Current Alt Cruise speed when at 20,000ft gives you almost the same estimated range, but in actuality probably even more because you get to power back sooner.