Nice rabbit hole.
Probably it will come down to Android being a mess, but I’m no developer and can spout something unspecific like this while getting away unpunished 
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I’m under the impression Android itself uses ext4, so in general the support is there.
Android doesn’t easily allow using ext4 on an SD card, but smart people got this to work in the past.
https://jplitza.de/blog/2013/07/04/external-sd-card-with-ext4-under-android-cm10-1.html
The more recent example, however, ends with “Hey just updated to LOS18.1 and this approach seem not to work anymore” … which leads me to this unsolved LineageOS 18.1 issue …
https://gitlab.com/LineageOS/issues/android/-/issues/3684
And just to say …
/e/OS is based on LineageOS.
So for the moment it seems to me that you can’t have nice things, at least on Android 11 (LineageOS 18.1, /e/OS R).
Feel free to open an issue in the GitLab to discuss this with the /e/OS developers … https://gitlab.e.foundation/e/backlog/-/issues