he illegally removed multiple documents and then disseminated them in (at least so far) 4 different unsecured locations, including even his garage! Further, he insecurely stored some of these documents at a Think Tank which receives financial support from China, one of those documents was found to have been free and separate in another room meaning that it was being examined by someone without clearance, potentially spies. Joe Biden himself has expressed his own view of the matter of taking home classified documents after retirement "How could that even be possible". "How could anyone possibly be THAT irresponsible?" He indicates that his own action here is beyond a level of irresponsibility that he can even imagine. This goes beyond simple negligence and qualifies as "gross negligence". I agree with The Big Guy.
Also, a previous point ignored by you is that Biden did not answer truthfully about having completely turned in everything after the first batch, the second batch, and the third batch because in each case, more turned up later (unless we have a corrupt and incompetent AG who does not ask "Is this all of it?").
What will be particularly interesting is what documents he felt were worth all of this effort. We have reports that some of them involved the Ukraine at the time Hunter was there with Burisma taking part in his family enrichment activities. These details need to be investigated thoroughly, as does the list of visitors to Bidens home that could have had access to these classified documents.
Link: https://twitter.com/i/status/1612598929345941506