Would it be possible to increase or abandon the 400 char per "cell" and 2MB overall limitation? What would be the correct place to discuss this?
Topic on Help talk:Tabular data
It is definitely not possible to extend the 2MiB overall limit, no; that's a fundamental limit from MediaWiki. If you're dealing with that much data, this simple system is probably not right for you.
For the cell limit, I'd suggest a discussion on Commons about making such a change and then filing a Phabricator task asking for the change.