edit :
Alternative to boost regulators would be charge pump regulators that can double the input voltage with up to 97% efficiency ... so for example 2 x 1.2v-1.35v = 2.4v-2.7v for rechargeable -> 4.8v-5.4v which is fine.
The problem is when user adds two fresh alkaline batteries that may be up to 1.65v each but even if they're not fresh you double 2x1.5v = 3v to 6v and you have issues because most microcontrollers support maximum 5.5v
end edit
Yeah, if you want lowest power consumption, a LCD screen would be lowest power and a lcd segment driver can be very cheap.
You could also use an actual led driver that would allow you to set the maximum current and then multiplex the digits and give you brightness control so you could reduce the current as much as you want.
For example, this 14 segment 8 digit lcd display is only 3$ at Digikey and it's transflective, meaning it will be quite bright with regular light :
https://www.digikey.com/en/products/detail/varitronix/VIM-878-DP-FC-S-LV/1118603The reflective version is also cheap at 4.6$ if you get only 1 :
https://www.digikey.com/en/products/detail/varitronix/VIM-878-DP-RC-S-LV/1118605They get cheaper at 10pcs, around 1$ off each price.
Here's an example of a reflective display, with 4 digits, which allows you to have a couple leds on the sides and provide backlight if needed :
https://www.digikey.com/en/products/detail/lumex-opto-components-inc/LCD-S401C52TR/469783Both of these work with minimum 3v but you produce 3.3v anyway with your boost regulator.
Some yellow/green/amber leds would run direct from battery (2.4v to 3v) or from 3.3v and you could simply have a button user could press to turn on light for 10-30s and then auto shut down to save power.
The 8 digit displays seem to be 32 anodes x 4 cathodes , the 4 digit one naturally less
Here's loads of other segment lcd displays:
https://www.digikey.com/short/qvtfj58qexample of lcd digit drivers
32 x 4 :
Holtek-Semicon-HT1621B
https://www.lcsc.com/product-detail/LCD-Drivers_Holtek-Semicon-HT1621B_C7873.html or
Titan-Micro-Elec-TM1621B
https://www.lcsc.com/product-detail/LCD-Drivers_TM-Shenzhen-Titan-Micro-Elec-TM1621B_C2980111.htmlGN-Semic-GN1621CB
https://www.lcsc.com/product-detail/LCD-Drivers_GN-Semic-GN1621CB_C265477.html36 x 4 :
Titan-Micro-Elec-TM1726
https://www.lcsc.com/product-detail/LCD-Drivers_TM-Shenzhen-Titan-Micro-Elec-TM1726_C2937431.html32 x 8 :
GN-Semic-GN1622A
https://www.lcsc.com/product-detail/LCD-Drivers_GN-Semic-GN1622A_C2758336.htmland loads of others here:
https://www.lcsc.com/products/LCD-Drivers_584.htmlexample lcd drivers on digikey :
https://www.digikey.com/short/p9mq7885If you want LED based digits, you could use proper dot matrix drivers to limit the maximum current, and then have the driver adjust brightness per led.. the problem is the quiescent current of some of these drivers is quite high at 1-2mA
For example
Lumissil IS31FL3738
https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3738-ZLS4-TR/14308389Easy to use TSSOP package , 8 sources, 6 sinks , so ideal for 8 digits with 6 segments or 4 digits with 8-12 segments, 2.2 mA quiescent current, works down to 2.7v
Lumissil IS31FL3746B -
https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3746B-QFLS4-TR/12675543 -
It does 4 sources (anodes) x 18 sinks (up to 72 leds), so you could have 8 digits each with 9 leds or 4 digits with 14 leds or something in between. Quiescent current is average at ~ 2.3mA and can run down to 2.7v
Lumissil IS31FL3728 :
https://www.digikey.com/en/products/detail/lumissil-microsystems/IS31FL3728-QFLS2-TR/5319755 It can do 8x8 , 7×9, 6×10 or 5×11 ... quiescent current is high at 4mA, but shutdown current is uA worth of current, and can run with as little as 2.7v