My hair was in that kind of state too - really dry and brittle. I actually grabbed Olaplex a couple of times, but eventually the price just hurt too much. Honestly, I can't say whether the stuff is really that much better than cheaper alternatives. The results were okay, but not so dramatic that I could justify it.
There are definitely cheaper products that work similarly - a lot of people swear by brands like Kérastase or Schwarzkopf Keratin, which also have bond-repair technology but cost way less. Even at dm or Rossmann you can find cheap versions now that at least help with straw-like hair. The thing is: with really badly damaged hair, probably no conditioner alone will cut it. What matters more is taking a break from regular straightening and coloring, or at least doing it way less often - that does more than any expensive product.
What I actually tried and what helped more: doing an intensive treatment 1-2 times a week instead of daily, plus not forgetting heat protectant when styling, and blow-drying instead of straightening. After a couple months it was way better than with Olaplex alone. Try a cheaper alternative first - if it doesn't do anything after four or five weeks, then skip the expensive stuff and go to a hairdresser instead, they might also be able to give you some cut recommendations.