it’s totally fine to discuss this here.
the sd-card will probably not supported soon (or ever) by delta chat directly. there are lots of issues with that, among them security problems.
however, newer os support to expand the internal memory with the sd cards, and, of course this is also supported by delta chat then.
but, again, a function for cleaning up is already planned, you can track this issue here: https://github.com/deltachat/deltachat-core/issues/244