Personally my OS (Mobian) is running on the SD card (on a Pinephone, so onboard memory is rather inadequate). I have space but as my history grows the app is getting slower, taking several seconds to load a chat.
Would having a DC copy on your desktop be useful to you? Or storing mails as Maildirs to view them in the threaded view of another mailclient? DC’s use of open standards does make it more hackable.