If you try a magnesium battery, please tell us how well it works! Up to now I know them only as sacrificial electrodes supposed, meant, and claimed to protect steel from oceanic corrosion, but I've proposed to power underwater drones with a battery where seawater is the electrolyte, the other electrode is essentially durable and leached by flowing seawater, while the magnesium electrode is replaced before the drone dives to a new mission. Big autonomy, cheap, safe - if it works.
I suppose magnesium isn't used commonly in batteries because it corrodes even if no current is consumed. If the electrode is stored dry and inserted before the drone consumes permanent power, self-discharge is a lesser worry.
Maybe purity is an answer to the supposed self-discharge of magnesium batteries. Or some additive in magnesium, like Hg was added to Zn before Zn purity solved the self-discharge problem.
I admit a strong tendency to overthink. But in case your battery can't power the maxi-motor for the 30s, consider a depolarizer.