An even more lazy approach is logging into the console,

regardless if textmode or framebuffer,

and simply do upward arrow & enter,

since whatever starts your session is the last entry in shell history anyways :-)

This way there is less surprise/annoyance with flickering/crashing X,

like you could have when starting X unconditionally after login,

which could happen after you modified something to test.

On another console, to not spoil the history for that upward arrow & enter thing ;-)

Which works regardless of shell/WM/DE/OS/whatever, at least it does so for me.

Is this an approach that works well with screen locking, or is a display manager really necessary for that feature?

You don't need display manager to lock screen in i3. Just run or bind a key to run i3lock (or other lockers like slock)