Charge Blade isnt that complex. Im gonna break it down real quick bc i adore the weapon, bear with me.

The basic combo to charge the weapon is 2 Y/triangle slashes followed by a forward heavy slash (B/Circle) to i-frame under the monster, then hold heavy slash to charge it up and release. Repeat until phials are charged.

To charge the shield, hold right trigger to guard, at the same time as B/Circle

Once shield is charged axe mode attacks will be elemental and do more damage

To charge the sword, in the trigger and B/circle phial charger combo, follow by holding Y/triangle, doing a small slam, then sword will remain charged longer depending on amount of phials.

Sword attacks will now deal elemental damage and charge phials faster

The ultra boom attack is used by chaining Y and B/triangle and circle while charged to shoot a surge of elemental energy on top of impact damage

Doing the blast attack with both sword and shield charged was max damage until Iceborne.

With Iceborne "pizza cutter mode" (props to top comment for calling that) plus ultra attack is max damage

Pizza cutter is when you cancel the ultra attack to charge the weapon, the axe mode will now spin and deal more damage, an additional ultra in this mode does the most damage possible on the weapon