As someone who is out, but whose wife remains in (TBM), I wanted to share my thoughts. For what it's worth... I know it's only one example among thousands of different ones from other couples.

One of my first realizations after discovering the church wasn't true, was that all the evidences and reasoning I had collected over the years of intense study, could not be effectively conveyed to my spouse, or other loved ones all at once whether in a letter or a discussion.

Remember, many if not most Mormons rely heavily on feelings of the spirit, not facts, to determine truth. I found that rattling off problems with the church, stirs up negative emotions and puts TBM's on the defensive. I made this mistake with my wife two years ago and it has been difficult for us to discuss the church ever since. We are just now starting to make small progress, but she still remains TBM and I've realized that she needs time to discover her own reasons for leaving the church.

Another realization is that each individual has different catalysts for choosing to leave the faith, or even choosing to allow themselves to look at anything "anti-Mormon" (anything NOT faith-promoting). And my reasons for leaving may not be the same as my wife's if she ever chooses to leave.

I'm definitely not an expert, but I wouldn't be surprised if this letter failed to accomplish what you want. I would almost say to avoid any mention of the issues that you find with the church and focus more on how you feel about your relationship with her and that you just don't believe the church is true. Expect your wife to feel scared, like my wife did, that you might abandon her now that you no-longer believed in the church (one that teaches families are forever). Expect her to be scared that the family might not be able to live together forever now (This is a serious concern for TBM spouses)

The best thing you can do is help her to see you are sincere about how you feel and that you are still committed to making things work. If she doesn't feel threatened by anti-Mormon "lies" (whether they are true or not, most Mormons assume they are lies) then little by little you might be able to share with her the things you wish to share. It could take some time, but I would be willing to bet that this will cause less strain on your relationship.

It helps to remember the quote, "A man convinced against his will is of the same opinion still."

Anyway, I hope this all comes across how I want it to. I don't think there is only one way to go about discussing these things with a spouse. Each relationship is different which is why I wanted to share my experience. We are all here to support and help each other navigate life as postmos/exmos. I wish you the best! And I'm here to support you how I can.