That is how most people are doing it. BD on the 4th day. I think it is also ok to BD on the 5th day but it will lower chances of conception.
I do not think you should wait till you have fertility signs to start the BD. It is best mentally to get settled into the pattern beforehand and also in case you O without having many/any fertility signs that month.
I have a funny feeling you may want to try one month wiht one attempt just to see if it can work for you. You don't HAVE to, but just from your posts I feel like you may have a lot of worry about going to e4d if you don't at least try one attempt at pos OPK. It is entirely up to you, it is just the vibe I get from reading your posts. I got my daughter with every 4-5 days (just accidentally, we did not know that was something that could work then)
Yes, you can hold off to test with the opk but be aware it is possible to ovulate without having any fertility signs.
I haven't seen a pattern per se, it is just that the results with exercise have dropped after having stayed consistent and this lines up with the point in time people started doing the 4-5 days (and some of them did even less exercise, even though I did not want them to, so that may have some effect). 70% is still very, very good chances and I do not think it wise for you to exercise any more than 4 days a week anyway.

