I have a few AXS bikes and rotate through a stock of SRAM, MRX, and Bigrock batteries. The Bigrock batteries do not seem to last any longer than the others. I speculate that they are rating the battery based on the combined capacity of the two 3.7v cells inside it, rather than providing the total capacity at 7.4v as SRAM does.
If you're experiencing range anxiety, why not just bring an extra battery? I keep a charged battery in the downtube compartment of my GR201 and just swap it out whenever I get a low battery warning on my head unit.