Doors – Exterior door mounting plate – replacing door hardware

You could put some shims/wood filler/bondo in there, but it's 90% cosmetic. (The remaining 10% is that it would resist an attack with a prybar, but realistically, if someone is coming at your door with a prybar, you're doomed.)

The screws (assuming they're long enough to bite solidly into the framing), will do what they're supposed to do with or without filler.

Lockpicking is tremendously sensitive to the accuracy of manufacture. A good-quality cylinder really is more secure than a cheap equivalent. (There are good-quality second-source cylinders, but don't expect to find them in a $3 lock set.)

Degree and kind of protection from brute-force attacks varies; that's a lot of what the lock grading system is about. Grade 2 locks are physically more durable than the cheap grade 3 versions. (Grade 1, commercial, is more so but is mostly needed where you're also concerned about how well the lock will stand up to the wear and tear of being used frequently.)

High-security cylinders do make a difference. So do higher-security strikes, anchored to the framing rather than the trim.

Master keying should be used only when necessary and should always start with a more-secure-than-usual cylinder. (Some locksmiths describe master keying as "the controlled destruction of security" -- I'd say reduction but they aren't far off.)

And so on. Seriously, the number of ways to get security wrong or right approaches the number of lock designs on the market, and there are real limits on what can be discussed in a public forum .... or should be, anyway.