Too many large issues for one law. Personally all laws should be voted on individually with no riders, and no law may be written that has more words than our constitution. This is what would allow the entire country to read and understand every law written, as they are voted on and approved. Also sets up for the legislative branch to be a service position that ANY American can hold.
We had limitations to those with mental illness purchasing guns, until Trump revoked it. Also you have to quantify they type of threats made on social media. How many times, as a teenager, did you say, "I'm gonna kill you"? Of course back then, it meant to beat someone up, and probably still does for the most part.
Voter Fraud is really a non issue and not statistically relevant. If you want to prevent people from trying, make it a federal crime, that revokes most voting rights there. 2yrs would seem like the max for a crime like that. Having to show an ID when you vote is a must.
For Immigration, the FBI would not have the resources to background check every potential immigrant. Fine with the not letting in criminals, but the crimes and timing since arrested have to be defined.