NOTHING beats training and leaning to use the iron sights FIRST and PROFICIENTLY.
Once you have the iron sights down pat, then the rest is gravy, and DOES have it's place,
But Iron Sights are first and foremost, and you should train with them every time you are at the range.
As I get older (50 now) I know my eyes, reflexes, and everything else is tapering off.
I can no longer focus in three planes (Rear sight, Front sight, Target) so I use a lot of optics,
But I ALWAYS practice with iron sights just to keep my eye used to them.
Anything/Everything else can fail, but your iron sights will ALWAYS be reliable and accurate if you practice!
What you should know is the difference between No Magnification (1x) and Magnified optics.
1x will NEVER have parallax. Parallax occurs only when you have magnification, and is the cross hairs not being perfectly aligned in the center of the optical column.
Holographic sights (Red Dot) usually will correct for eye misalignment behind the center line of the optic.
The red dot will appear to move around WITH the target when you move your eye around behind the optic.
That is the reason our troops are doing so much better with 'Red Dot' optics, they don't have to take the time to center the eye behind the optics for precise shots.
Red dot on the target, pull trigger, usually a hit.
When you are using a 'Scope' with magnification, Parallax will make the target appear to move around behind the cross hairs,
OR,
The cross hairs will appear to move around on the target.
When shooting with a 'Scope', you MUST center you eye in the tube to make a hit.
Parallax adjustments are there to solve that discrepancy.
Parallax adjustments correct the apparent movement of the cross hairs or target under the cross hairs when you aren't exactly aligned with the tube.
NOTHING will substitute or correct for bad shooting form!
If you have the comb of your stock welded to your cheek bone,
So your eye automatically centers in the tube,
Then you will cut out a lot of the 'Parallax' problems that aren't really Parallax, it's bad shooting form and ill fitting firearm.
'Red Dot' holographic sights take that issue out of the equation when they are 1X, but if you go with magnification, you will find there is 'Parallax' creeping back into the sighting equation.
I can't stress enough to have an appropriately sized rear sight aperture (hole size in the rear sight) and practiced shooting skills with the iron sights!
The military drilled into us proper sight picture and cheek weld, and we were able to hit targets I didn't think possible.
There are people shooting out to 1,000 yards with aperture sights and scoring good groups, so it's VERY possible.
Just as important,
Learning to keep the rifle VERTICAL.
Keeping the optics/sights directly above the barrel!
Look at your watch,
The 1 minute mark just past 12:00 o'clock is 6 degrees of cant.
That's a 1/2" miss at 100 yards,
55" at 1,000 yards!
Practicing with a bubble level (Spirit Level) on the optics will teach you to shoulder the rifle SQUARELY AND PLUMB, and will take that cant out of the equation.
When I got a spirit level on my 'Gopher' rifle, I did a LOT LESS MISSING of those little prairie dogs!
A 1/2" miss gives those little devils a whole new lease on life!
That carried over into the Military shooting I did, and it increased my hit rate more than I can express!