I understand the desire to write the letter. I DID write a letter outing wrongs, stating facts and listing what I contributed and what he still OWED me. fair is fair...except in love and war. It took me about a month to draft and was over 6 pages long (single-spaced). I followed up with an email, my dad followed up with calls and letters. The response? Nada. Nil. Nothing. His ego was perversely fed and he then knew the things he needed to justify or improve his lies about. As Hawkeye wrote, "I tipped my hand."
You have the court system to work through. Use it to YOUR advantage. Don't let them think you are the unreasonable, difficult, spoiled ex-wife, sell them the story of the articulate, hard working, do right, nurturing mother. I would use the sympathy card there like no other. IF that means that you are waiting tables in a coffee shop to show how you are trying to make ends meet and your income is still not enough to buy the kids clothes and braces, but he has money to do fancy things x, y, z, I think you make a better case. Court is not about what is fair, court is about how you market yourself using facts. List the facts. All the good you have done, all the bad he has done. Not emotions, facts. Not opinions like this is right, this is wrong, what are the facts? What did the kids have before the split? What do they not have now?
I want you to win, Pippi. For the last several years, appealing emotionally or rationally to your ah hasn't worked. Will it work this time? I doubt it. History says it won't. The letter will give him ammo about you, your desires for $, kids, etc... Don't hand him the blue prints about to how to screw you again.