The number one sign of cheating is hiding/guarding the phone. The number two sign is the change in behavior - being cold and distant, maybe annoyed by everything you do.
So you have the top two signs.
Other tops signs are new lingerie that are not being used with you. Have you asked her about those?
Another top sign is losing weight. And exercising. But I would lay off that as far as asking.
The most common affair partners are co-workers or ex-lovers.
Another top sign is new grooming.
So ask about the lingerie, and ask about the lack of intimacy, and tell her you are suspicious and add up all the things you listed here. Normally I would say don't show you are suspicious, but she already knows, with her smirky response to your first request about the phone. Let her know that you love her, and ask her what is the problem lately, what have you been doing or not doing.
Ask specifically what does she talk with the ex-boyfriend and ask to look at their communications. You know they are in contact with each other, so unless she is deleting everything, she will be able to show you a full, rational conversation via text or email or some other social media. If it is deleted or partially deleted, then you can follow that lead.
You have legitimate reasons to think she might be at least considering cheating. If you lay it all out, she should see this. If you are low-key and calm and laid back about asking, in a tone of worry about a loved one, her response should be one of "I understand why you might think that" and then she will try to explain the reason your suspicions are not founded. She would want to assure your worries and be more open, and possibly step up to improve the marriage.
Also, you can tell her you want to improve the marriage, you feel a distance and a problem and it is not something you want, you want to share her life as an intimate partner, does she not want the same? And if so, what should we do about it? These are things to just see where her head is at.
In the meantime, besides texting, see if you can see her browsing history, where she is exploring on the internet, you might get lucky with searches related to cheating or hiding it.
Also, I don't know how far you want to go, but savvy tech cheaters usually don't consider that a voice-activated recorder might be in their car or in the spot in the house where they talk when the loyal spouse is not around. They are focused on deleting and hiding the phone, not the relatively low-tech voice recorder. The car is usually considered a safe haven, no one can hear what is said except the occupants (unless you put a voice-activated recorder in there). If not the other man, you may catch a call form a confidante girlfriend.
But I think she is cheating now, and planning to consummate, given the other signs combined with the new lingerie that you don't get to see.