There was clearly a lot of nuance that we were missing. I do think there is a good chance your husband doesn't think about how those meme's would seem to you 1) because he's posturing to the boys 2) He doesn't think you'd see it. I think it is likely more a symptom of low self-esteem and a need to be accepted by other men (in a stupid, immature way.)
As I've mentioned I had to walk my husband through what it felt like to hear him say a stupid "man" joke, etc. after DD. I had to help him learn to put himself in my shoes and how it would sound/look to me after having been betrayed so deeply. Things that would have been funny or only a bit off-color were suddenly full-fledged triggers.
Through MONUMENTAL effort, I kept my calm and walked him through what it be like if the shoe were on the other foot. Once he started imagining HIMSELF as the betrayed one and him having to hear me say similar jokes in front of our friends and family (ESPECIALLY those that knew), he started to "get it". It makes him sound stupid - and he's actually a very intelligent man - just not about emotions. But he's also a quick learner when motivated and after this happened 4-6 times over the first year and we discussed it, he kept getting better and better at seeing it from my perspective and being able to stop the joke from even forming in his HEAD, much less saying it out loud. I can't remember the last time HE triggered me. Now, if I am triggered it is mostly about outside things (places, etc.)
It appears (if I've understood correctly) that all the "flirty" texts he sent were in the pre-DD era. While it definitely shows a pattern of behavior - it also tells you more about who he was then. People do change. NOT all by any means, and there is reason to be VERY, VERY cautious in creating a future with a WS. But some people do change - significantly - in response to seeing the harm they have caused themselves and others. For instance, I know I am not even remotely like the woman I was 10 years ago. My values have changed (I put family and relationships above work and success now) and I am FAR more vulnerable and compassionate with friends and family. I value honesty more than "making things easy" - and have had some very difficult conversations that I would have avoided in the past. I also know real change happens because it is something you want for yourself, not because you are forced to by others, so it is important to gauge how motivated THEY are to change. They don’t have to do everything perfectly, but they do need to be trying – regularly – not just when the pressure builds to a point that you explode. I saw a quote that said, "We change when we are tired of our own bullsh*t." I would agree with that.
You seem like you want to give your marriage a chance and he has been doing more than we were aware to try and show you love and care. But not enough.
So now it is on you to be CLEAR and unwavering about what you need and expect. Do not assume he knows what to do - or even how to do it. Look at his parents - that is the emotional IQ he was raised with. It was probably limited at best - and most men were actually told by words and actions NOT to spend time on emotions and self-reflection. It was considered "girly", "weak" and a "woman's domain." Dumb, but true.
So unless he is was of those exceptionally rare individuals who decided to place value on gaining a higher level of emotional intelligence and skills after leaving home - that is STILL his level of emotional IQ. It doesn't make it okay - it just IS. But it is perfectly within your rights to demand that for you to stay and rebuild - he needs to learn a lot more - ASAP. This is one of the places therapy and coaching can really, really help.
I also think you need to grow some ovum (in place of balls that wouldn't suit you, lol) and start having the hard discussions. By your own admission this is something you have avoided. But it is keeping you stuck, and you’ve clearly come to the same conclusion yourself.
The night you take his phone, I’d have him hand it over – but before you leave with it – give him a last chance to come clean. Have a list of prepared questions that you want answered – with some open ended ones as well (for example: Is there anything that I haven’t thought to ask about that YOU KNOW would upset me? Any lies of omission that are still there because I just haven’t thought to ask about it?)
My suggestion is to use as much compassion as you can - it will help keep him from going into fight or flight. Remind him that you HAVE seen positive changes and that they are the only reason you are still there or why you still have hope that you guys can get past this.
Remember that your goal is to get to the TRUTH – and to do that, he will need to have reason to believe 1) that it is REALLY the only way forward 2) that it is safer to tell you the truth than not to.
Since you have no clue what these things are that he feels are so “awful” that you will immediately leave him – you have no way of knowing what your reaction will be. So rather than try to calm his anxiety by assuring him it will be okay if he tells the truth (because it might not be), instead explain to him this:
“We are at a crossroads. In order to move forward I need to know the truth of what I am forgiving. I need to know that I am safe from some new awful piece of information dropping out of the sky when I least expect it. To that end, I have some questions I need to ask. Do not say the first thing that pops into your head – really THINK about these questions and answer them truthfully. I will wait if you need a moment to compose yourself. I don’t expect that this will be easy for either of us.
IF you tell me the complete and unredacted truth to any questions I ask – AND OFFER UP any information that I don’t ask but YOU KNOW would impact my desire to stay with you – we MAY have a chance at staying together. I know that every impulse in your body is telling you to lie right now. Your head is telling you to shield me by lying or telling me the smallest amount you possibly can. But let me assure you – that is the absolute WORST thing you can do right now.
The truth is, I cannot guarantee that I will stay when I know everything. But I CAN 100% guarantee that if you lie or “forget” to tell me something important and I EVER find out about it from anyone other than YOU – I will leave you. Not because the specific act – but because YOU weren’t the one to tell me. It is the SUREST path to the 100% destruction of this marriage. Please believe me when I say that and take a minute to really let it sink in. Yes, I MAY leave you over what you tell me – but I will DEFINITELY leave you if you aren’t truthful. Telling me – coming clean - is the only way you CAN protect me now. The deeds are done and there is no taking them back. BUT you CAN protect me from finding out from someone or somewhere else. You CAN protect me from the devastation I would feel if I rebuilt this marriage on a lie and found out 10, 20 years from now that there was more. Truth NEVER stays buried so don’t make that mistake again. Stand in the light, come clean and let’s see if together there is still a path forward.”
Then let him think about that for a minute before you start. Consider stopping and doing deep breathing exercises both before you start - and several times during your talk. It will help you stay calmer and more present. At the end of the talk, tell him that you realize that this is scary and hard – and that he may realize that in a day or two that there is something he INADVERTENTLY left out. Stress CAN make this happen. Let him know that if that happens and HE seeks you out to clarify or add something he forgot – you will understand that this was an unintentional oversight. BUT again – if you find out things through any other source – your digging or someone else – all bets are off.
Have the conversations you need to “clean the wound” as Keep suggested. Yes, he will likely get depressed and down for a while - that is a NATURAL consequence of self-reflection when you've done something awful. I would be more worried if he DIDN"T.
Your plan is a good one and may help you put some of these questions/doubts to bed in your mind.
I hope some of this helps and that you get what you need to make an informed decision. Feel free to PM me if you need anything.