iOS Mail cache doesn't clear when no accounts registered.

On iOS 15.8.2 running on iPhone SE 16GiB, I have Mail set up to read my multiple email accounts. Being 16GiB, space is at a premium, and new mail won't come in unless I free up space.

iOS is already taking a big, fat 7GiB for itself. The other major space-hogger is "System data", a black box that contains everything that is not a picture, not an app, and is 4.5 to 6GiB, depending on the day.

To help clear it, I use the well-known but crude and (very) time-consuming trick:

1. Turn on Airplane Mode, set time +1yr ahead from the real date
2. Wait for hours until space clears
3. Restart
4. Set time +3 months ahead
5. Wait for (hopefully) an hour until space clears
6. Restart
7. Set time +1 month ahead
8. Wait
9. Restart
10. Set time to proper time, turn off Airplane Mode
11. Restart again

On the other hand, I see that iOS' Mail has a tendency to hold onto an obscene amount of cached data and never clear it. Now I know that cache is important for performance purposes, but there's no way iOS would need to hold onto 350MiB of cached mails. As a comparison, my professional and rather busy Outlook app holds 30MiB of data with 300MiB of (obese, given the functionality) app.

Already tried to clear email cache using the following methods, to no avail:

1. Erasing the email accounts: works, but doesn't clear anything
2. Offloading the Mail app itself: doesn't really work. Cloud with a down arrow appears next to Mail in the settings and it disappears from the home screen, but Offload button doesn't change to Reinstall.
3. Uninstalling Mail completely: this should remove all app+data, but somehow doesn't work with Mail as data is retained.

Won't do as problem will likely reoccur:

Configure iPhone as new

Any hint on this issue¿