What is the approximate battery discharge rate of Google Pixel 3XL phones when turned off?
The answer starts from what you asked towards the end of your question.

Li-ion battery's are charged to around 40% while storing. That's about the charge level (give or take few % points) you'd see when you take a phone out of box for first time. Reason


Lithium-ion must be stored in a charged state, ideally at 40 percent. This prevents the battery from dropping below 2.50V/cell, triggering sleep mode.

Sleep mode is a protective mechanism to prevent deep discharge and you can't charge the battery conventionally when it enters that mode.

But batteries do self discharge and the typical rate of Li-ion batteries is about
2% per month (Table 3). But this does not mean you get a fully discharged battery say after 2 years. That's because


There is virtually no self-discharge below about 4.0V at 20°C (68°F); storing at 3.7V yields amazing longevity for most Li-ion systems


But yes, your battery would suffer a capacity loss as you can see in Table 2 of link quoted in above point. If you can get a new battery from OEM, which is manufactured recently, this risk can be mitigated and you can use it for long.
But if your use case is only photography, I guess you need not change battery at all.

This is generally applicable to all Android devices and  your specific model should not be an exception.