I plan on using both passata and paste (which I didn't use before).
Two hours should be enough for the flavors to combine and the meat to get tender. Why do you suggest not using passata with that cooking time? Too watery? Tomato flavors not condensed enough? I mean, I'm adding wine and some water anyway. As long as I cook for 30-60 minutes without a lid on top I should be good as far as consistency goes.
Now I'm undecided with the milk, however.
The "butter on pasta" thing sounds counterproductive to me since I want to sauce to stick on the tagliatelle as much as possible.
Yeah passata is too watery and smooth; it will take a long time to evaporate and condense and you want small tomato chunks here and there!
You may use "pelati".
30 minutes is not enough, let it cook for 90 at the very least. Onion and other vegetables should practically dissolve.
Do not add water right away (or ever in fact, unless you realise it's too dry and going to burn): you want your meat to brown a little, with just oil and soffritto at first. Adding water, or too much liquid at this time, will make your meat boil. Eeww! You don't want that!
This is also why you add wine in small amounts and let it "sfumare" (evaporate quickly) at high temperatures. The pan should be very very hot at this time.
If you really really feel the taste is too..harsh? I guess you may add a little milk but not in the sauce itself; instead stir your pasta with the ragu in a hot pan (as in, pasta should finish cooking in the pan for one or two minutes) and add small amounts of milk and some of the water you used to cook pasta.
But If you're using more carrots the Ragù should be sweet-ish and gentle enough and it shouldn't need anything else.
A small amount of butter won't make the sauce not stick to tagliatelle, it will just make everything a bit smoother. Some like that, but most prefer their Ragu a bit more...stingy?
Let me know how it went, I'm off to cooking some Ragu myself; thread inspired me.