Firstly you should keep using silicones in heat protection any time you use a flat iron, they are king for this purpose. Also use a low heat setting and only on bone dry hair, preferably dried the day before. Consider wet wrapping for heat free straight hair, consider giving your brush and flat iron to a friend or relative for a few months if you cannot trust yourself.
THE NATURAL HAVEN: Straightening Facts :How hot is too hot?
THE NATURAL HAVEN: Blow Drying Reduces Moisture in Hair (even with low heat!)
THE NATURAL HAVEN: Straightening Hair - Relax or Heat?
Be aware that every time you flat iron you set yourself back, flat ironing temporarily breaks the protein bonds in your hair, eventually this can become permanent (straight bits). It's unlikely you will be able to get healthy looking defined wavy or curly hair whilst you continue to both bleach and flat iron. My hair takes up to three washes to recover from being worn straight (no heat), people who flat iron it can take weeks or months unfortunately.
Ultimately the best thing to do would be to choose between beach blonde OR artificially straight hair, you should be able to have healthy hair with one of these. You might be able to do both if you only lift a small amount from your natural colour, you still look to be very light. It would likely be gentler to have a keratin straightening than fry your hair with the flat iron regularly.
Hair is dead so cannot ever be permanently repaired, you have to grow or cut out the worst of the damage. Some ingredients can help patch repair (temporary) damage, increase strength and elasticity, even help protect from bleach damage. These include coconut oil, hydrolysed protein, ceramides and panthenol.
That conditioner may not be suitable for co-washing, the second and third ingredients are a fatty acid (oil) and a humectant (attracts water), you need a product rich in fatty alcohols (only the fourth and fifth ingredients) and cationic surfactants (barely any) to cleanse hair. Some polyquats build up so watch for them. I am in the UK and use Inecto, you might be able to get that elsewhere in Europe?
Ingredients Commonly Found in Hair Care Products
You probably need to use a LOT more conditioner especially leave in on very wet hair to get your curls to form. A little conditioner will just vanish on very porous or damaged hair. Limit humectants which can make hair frizz and poof, use more occlusives (oils and butters) - these basically replace what silicones used to do for you.
I hated my hair when I started, and mentally 'wrote off' the first year for growing out damage. Now I don't have perfect hair (I still dye but roots only), do have waves and even ringlets I never knew were there. For me the lightbulb moment was realising I would NEVER have good hair if I carried on using commercial products and standard styling techniques, it didn't give me beautiful healthy hair in twenty five years of adult life and a lot of ££££!! Something had to change if I didn't want to still be battling ugly damaged hair in a few years time. You are much more sensible than me if you have had your lightbulb moment after only five years.