Yes, REBOOT unlocks after you beat the final boss. Once bosses start at level 20, every 5 DEAD PIXEL kills brings up PIXEL ZERO. Beat it and you get the choice between REBOOT and CONTINUE, and after that REBOOT stays in the SYSTEM tab too.
You’re right about the slog, though. Saving up 40B for ARMOR PIERCING is a wall at the moment, and that’s the wrong place for it, right before the best part. Thanks for flagging it.