What kind of state or federal law do you propose, with the unlikelihood that the 2A won’t be repealed?

Easy, just do the same thing as other western countries. If the 2A is an obstacle, then change it. The unlikelihood of and changing the 2A is the heart of the problem, not an constraint to be worked around. The question is why are gun rights sacrosanct, but other rights can taken away just by saying 'think of the children'.

If you think the 2nd Amendment is bad now, just wait to you see what happens if we're unlucky enough to ever have a Constitutional Convention in our lifetime.

That’s not how amendments work

Don't look at school shootings, they don't matter. What matters is how many, not by what means. Note that the most deadly such attack was with a truck, not a gun.

The data proves nothing about whether it works because it's a cultural problem far more than it is a means problem. They choose to go out in a blaze of infamy rather than be a nobody. They generally are planned long in advance, there's plenty of lethal things they can get their hands on.