Legally, perhaps it could be sufficient to include a check box saying "I certify that I am the copyright holder to everything I upload". Compare with the way EU handles the cookie law and GDPR consent click-yes.
You're joking here, but a politician in Poland, when asked about how shall a popular link sharing platform (wykop.pl) deal with submissions, of which there are 100k / hour in peak times, actually suggested that "users should know not to upload illegal materials".
The last sentence threw me off (about certain content not being covered). If it was just your first paragraph, I would have thought it was sarcasm. And I was tired.
And that not all content will be covered (although they appear to have accidentally forgotten to include that bit in what they published).