I would probably just have you do every 4 day method in that case. Unprotected sex every 96 hours apart for the first couple months (start after AF ends and going through till you're sure you've ovulated even if that means going all the way through till your next period begins) and then go to every 72 hours apart.
I would not have you guys do abstain as it hasn't worked and your husband is 35. And if OPK are problematic for you then the digi's will likely be trouble too (they work very much the same) and it's just less stress to do e4d.