How do I fix 'Mail cannot save information about your mailboxes' error after upgrading to Sequoia on my M2 MacBook Air?

After upgrading my M2, 2022 MacBook Air to Sequoia (I currently have 15.4 beta installed), I get an error most of the time after the native Mail app has been opened saying: "Mail cannot save information about your mailboxes because there isn't enough space in your home folder". Once the error displays, I have to close the Mail app. I found an older article that encountered the same issue in 2019: Mail cannot save information about your m… - Apple Community. I tried the steps in the article (removing the .plist file and opening the Mail app again), but that solved the problem for one or two days before I saw the same error message again. I haven't tried the more advanced steps outlined in the article discussion about a corrupted hard drive. I'm hesitant to do that until I get more recent feedback to confirm the root cause. My MacBook Air has 8 GB of memory and 256 GB of storage, most of which is full. I downloaded the third-party tool "CleanMyMac" app and followed its instructions, but even after freeing some of the memory, I still encountered the aforementioned error. Can someone help me troubleshoot? Thank you!

[Re-Titled by Moderator]