It is the brand of OPKs that is the problem. I have not been able to find any good OPK strips here in Europe, they are all poor quality and low sensitivity and on charting websites, so many the European ladies don't get dark lines or true positives. I have personally tried that exact brand you are using and I never got even a light line when other brands showed at least something. But on some other European OPKs that work a bit better, my 'positive' OPK looks like the OPK on the left.
Bc the OPKs here are all so insensitive/bad quality, I also don't get much warning in advance of O and twice now I have Oed on the same day that I first got an OPK that looked even close to positive. (With the Wondfo brand I used in the US, that never happened, I always Oed the day after my first positive/dark OPK.) Like you I wanted to do one attempt at positive OPK and not BD every 4 days but I tried that for 4 months and 3 of those months I didn't BD until too late, bc the OPK turned dark at the last minute. Now I really wish I had just BDed every 4 days those months!
Where are you in Europe? The clearblue digital OPKs (NOT the monitors) are reliable but expensive, so I would switch to those for now. If you are in the UK, there are some supermarket brand OPKs that are pretty good, I've heard, one is Superdrug.