Not my daily driver (yet?), but made a custom dumbphone by getting a Nokia 3310 without a battery or charger and implanting a 2500 mAh Li-Ion cell and a USB-C port (details
Replacement Nokia 3310, 3390 Cell Phone Battery | Battery Mart
Nokia 3310 (4G) Battery BL-4UL - 1200 mAh - XtremPhone - Everyday Supply
Nokia BLD-3 - Cellular phone battery - Li-Ion - 780 mAh - for Nokia 2100, 3200, 3205, 3300, 6220, 6225, 6560, 6585, 6610, 7210, 7250 - Walmart.com