Power SIM800l gsm module from a 12v car battery

Hello all, beginner here. It would be great to hear advice on this matter since I have no experience whatsoever with choosing the right power supply strategies.

So, I'm trying to power Arduino nano(5v) and SIM800l(~4.4v) from a 12v ~50Ah car battery. I have two 3A step-down LM2596 buck converters. Now the thing is, is it reasonable or efficient to use both of them (one set at 5v for Arduino and its peripherals, and the second one set at ~4.4v for gsm module)?

Or perhaps to use only one LM2596 with 5v set at the output with 1N4007 or similar smd diode to drop a voltage for a GSM module (that would limit a current also I guess), or there is a third better alternative?

The nature of my project is that both Arduino and GSM module would sleep all the time except for a few minutes a day when they are going to be awakened (Real-time clock for wake alarm) to send measured data from sensors over GSM. Being able to last on a remote location for over 20 days is a priority, thereby I'm trying to find the most efficient strategy for long-lasting battery life.

I'm aware of LM2596's inner wasted current of ~20mA when the power LED is desoldered from the module.

And yeah, I'm aware of the idea that AtTiny85 with a MOSFET could be a great choice to turn everything on and off, but at this moment I cannot find them in my area and have no time for investing in learning them. ( Though the concept of powering AtTiny85 from maybe a few 18650s that would only count time and trigger mosfet sounds ideal since the entire project wouldn't waste any current from 12v battery for more than 99% of the time).

So yeah, no AtTiny for this time but any other advice would be useful and if I'm on a wrong path fill free to correct me, cheers.

I'd power an Arduino from the same voltage as the GSM module. Doing so does not only simplify the circuit design but also assures signal level compatibility. Or look for a module for standard 3.3V or 5V supplies.