Im coming into this really late, but i build decks with a $400 Dewalt. I also install stain grade trim with it, bc its perfectly tuned and i check it every time i move it. Gonna get the Kapex before Xmas, bc I deserve it, not bc i need it. I use my miter saw everyday, but never use DC unless im set up in a garage or inside the home. And yes you can use a CT 22 with non Festool saws, you may just need an adapter. Get a Forrest Blade for another $100 and call it a day.
What I cant understand is if you have all this nice equipment, how do you not already have a miter saw? I would consider it a must have for cutting anything long into smaller more manageable sections. Am I missing something about the work you do? Plus a miter saw without a nice extension wing set up is worthless when cutting 20' deck boards. Even the Kapex stand is way too small for that. My wings are nine feet each way.
Based on something Kevin Stricker said I am noticing people who installed composite early on in the game already looking for redos in CT, especially if there is a lot of canopy bc they stain and look horrible eventually. You should be more concerned about what material you are gonna use then what saw to get. And talk to Scott B about finishing! Eric