It's a terrible idea unless you are willing to lose the hair, you are doing at least three damaging things not two. Light brown from dark would need persulphate bleach not simply peroxide.

Hair is dead so nothing can truly repair it, good deep conditioners primarily help maintain hydration they don't fix the protein structure nor replenish the lipid cuticle glue. You might get away with it if your hair has been babied for some time, that most definitely does not include flat ironing. For a start with flat ironing you need to be bone dry hair or risk bubble hair, even a single session can cause irreparable damage.

Also consider how you will show off your dip dye without quitting or reducing protective styling. Mechanical damage and weathering are an issue for all longer hair but particularly chemically processed - sunlight, brushing, combing, rubbing on clothing/ car seat/ desk chair/ bed linen and so on.

If you are determined start super babying your hair in preparation lay off all heat styling instead practice techniques like wet wrapping. Choose products which are rich in ingredients proven to penetrate or temporarily patch up damage like coconut oil, hydrolysed protein, ceramides and panthenol. These may not be effective if you are using products heavy on ingredients that coat or build up like silicones.

You might consider having a detailed hair analysis from Komaza Care, they can tell you what condition your ends are in and if they think you could risk another chemical process.
Komaza Care HAIR ANALYSIS Service - Rave! - Long Hair Care Forum

Useful articles
THE NATURAL HAVEN: Straightening Hair - Relax or Heat?
THE NATURAL HAVEN: Heat on Wet Hair = Bubble Hair
THE NATURAL HAVEN: Straightening Facts :How hot is too hot?
THE NATURAL HAVEN: Blow Drying Reduces Moisture in Hair (even with low heat!)
Also check out the book 'Science of Black Hair' (preview on Amazon, few videos on YouTube).
2a-2c, medium texture, porous/ colour treated. Three years CG. Past bra strap length heading for waist.

CO-wash: Inecto coconut/ Elvive Volume Collagen
Treatments: Komaza Care Matani, coconut/ sweet almond/ fractionated coconut oils, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss Ultimate Volume, various Elvive
Styler: Umberto Giannini jelly, Au Naturale styling gelee
Flour sack towel, pixie diffuse or air dry.
Experimenting with: benign neglect