Best method to lighten my hair?

Adding another picture of my hair the last time it was blonde. I achieved this color at home from brown, so I know it's possible! Haha I just honestly don't remember how I did it 🙁

Getting to an all over blonde from highlights is actually pretty tricky. If you put bleach over everything the highlighted parts will be over processed before the darker parts have lightened enough.
Hi lift blondes are also bleach, they usually come with a very high level of peroxide and are only designed for virgin hair.
Any form of lightening is likely to cause damage and breakage on the parts that are already light blonde.
The only way to do it is to go through each section and apply the correct strength of peroxide for the colour of the hair, it will involve a lot of separating and sectioning. (and a ton of strand tests to get the strengths and timing right)
There is no reason a salon can't continue to add highlights to give an overall much blonder look.

Is the warmth in the ends caused by dye or is it your natural colour? If your hair has been dyed then I would suggest using colour removers to get rid of some of the old dye pigment.