"So, tackle the actual problem - not your wild leap of imagination into what the problem could be. Talk to hkm abojt the lack of closeness. Invest in the relationship, and try to tackle that. Arrange things to do together. Try to recover the closeness."
But what to do if he insists that there is nothing wrong, everything is fine, you're being paranoid?
You know the temperature of your marriage has changed but you don't know why. Maybe you are being paranoid. Maybe he's just madly busy with work. Maybe he's on the verge of a breakdown, or suffering from depression.
And so you plough on, hoping things will improve, hoping you'll come out the other side, unwilling to throw away many years and devastate your children. You suggest date nights but still something is off. You suggest counselling but he says there's no need.
At some point, imo, it is ok to look at his phone. Either there's nothing there and you can keep ploughing on, or there is and you can breathe a big sigh of relief for finally having an explanation, and a cast iron, guilt-free reason to end it.