Anytime bro. When you are short on budget right now, its wise to change battery at apple service center and continue using it. as you already have a pro device, it will be good for another 2 years atleast. So upgrading to base models should be considered only if the current unit is making you to spend more or not fit for your daily usage.
For calls, messages and apps usage. if 12 pro max satisfies then using it for few more months will be smart decision. calculate and decide bro.