This task should be either better phrased or declined:

• the part about completing the upload process is already discussed on T85561 for UploadWizard (Special:UploadStash is MediaWiki core, but the main user is probably UploadWizard) and there is already a patch about resuming stashed uploads in UploadWizard;
• the part about the Internal Server Error can be reproduced with any file larger than 1Mio, but it is a feature to avoid abuse with people using Special:UploasStash as a file drop, see comment in code.

Possibly the second part could be slightly improved by either removing the link in Special:UploadStash for files larger than 1 Mio either displaying a more friendly error message, and it is not really a 500 Internal Server Error but rather a 403 Forbidden (it is not a server error, but a refusal according to a policy).

I'm facing the same 500 internal error on a MediaWiki instance I'm working on. I understand the reason of this 1Mio limit, but this should be at least configurable in the LocalSettings to allow non-Wikimedia wikis to adjust it depending of their need.

Change 906772 had a related patch set uploaded (by 0x010C; author: 0x010C):

[mediawiki/core@master] Add UploadStashMaxServeBytes configuration setting

https://gerrit.wikimedia.org/r/906772