Like you, I have an old offline eReader (4th gen Kindle, replaced battery). My eReader lasts multiple books of use on one full charge. I also have a ReMarkable

Like you, I have an old offline eReader (4th gen Kindle, replaced battery). My eReader lasts multiple books of use on one full charge. I also have a ReMarkable 2. The battery lasts about 14-16hrs of on-time for me. I keep it in airplane mode and have sleep mode disabled to prevent it from locking after 40 minutes. I turn it on around 10AM and turn it off at 5PM, writing on it sporadically between those hours. The tablet reaches a low battery state by Wednesday.