Iv heard you can use opks as hpt is that true?

Thought I would use opks from 10 dpo and see what happens, as it's too early for me to use a hpt atm. ( I know the best way to tell is hpt but don't feel ready to take one yet just incase it's bfn as been trying for 21 months)

Iv only been doing it for the last few days as didn't think they would pick anything up after ovulation as never have before and second line there and slowly getting darker ( looks darker in real life not on photo)

I am currently 12dpo and been getting loads of pregnancy symptoms.

. Feeling tired

.headache on and off

.nausea

.bloating

.upset tummy

.consternation

.sore/heavy boobs more visible veins

.cramping

.lower backache

. Fitbit is all over the place with my heart rate my resting heart Rate is normally in the 60s but the last week it iv had 115 reading when I'm sat down ( read that fitbit can detect pregnancy as heart rate gose up)

And just not feeling myself