Remove minimum number of points from pattern unlock I've recently gotten a Samsung Galaxy J5. Its default (insecure) unlock procedure involves simply swiping anywhere on the phone in any direction. My primary concerns are (a) being able to quickly unlock and use the phone and (b) not accidentally unlocking the phone when it's in my pocket.

As it is, the 'swipe to unlock' feature is too easy to accidentally unlock, since any movement in any direction is sufficient. A solution to this would be to setup a simple unlock pattern that can be executed quickly and easily. That would make the phone much less likely to be accidentally unlocked whilst in the pocket. However, setting a pattern lock requires a minimum of four points to be used, for security reasons. Using a three-point unlock would make life easier as it would just be a quick swipe in one direction (crucially, a very specific direction, which would make the system much less prone to butt-dialing)...

I understand why four points are mandated as a minimum, but my concern here is not around security. Can anyone offer a solution that would allow me to either (a) remove the minimum number of points required for a pattern lock or (b) replace the lock screen with something similar that involves a more specific swipe than 'anywhere and in any direction'?

Note that as the phone is new and therefore still under warranty, I have no plans to root the phone unless absolutely necessary.
A quick google search for android custom lock screen presents me with a list of lock screen apps (for example this one: https://www.androidpit.com/best-android-lock-screen-apps). I don't think these apps need root access to function.

Many of these apps support "slide to unlock". Which is basically the same as a pattern consisting of a straight line, which is what I assume you want.
Most apps I saw require you to swipe from left to right on the bottom of the screen. This is a specific direction starting from a specific location on the screen, yet the pattern is incredibly fast and easy.
This prevents most cases of "butt-dialing" without giving up speed of unlocking your phone.

Take your pick from the many lockscreen apps that support this and enjoy! :)