It's too bad that I can't find the fully-translated book from the original "Fate/stay night" game anymore. I neither watch the UBW anime series nor the movie one, but I'd read every route from that book many years ago.I get what you're saying and that this is a different medium from the VN (which I have not read), but weren't the extra routes unlocked after finishing the previous one?
I'm still watching the movie and really the only thing that you absolutely should watch (or refresh your memory of) is the first episode of ufotable's UBW (episode 0 would be nice too). For anyone that still hasn't started with the movie: they only fill in the gaps on that first episode before literally SKIPPING to episode 2 of UBW and diverging from UBW route. You don't even get the summoning.
But I'm only in the middle of the movie.
Finished the movie. I thought it was a surefine episode of ufotable's Fate. The only thing I hated was the CGI fest in the city fight and random 2second overanimated dramatic shots (and I already complained about both when the trailers came out).
Episode 0&1 of UBW are absolutely recommended before this movie but other than that I think you're good to go. Some things will not make a big impact if you've not seen all of UBW but I don't think it's necessary as far as this first movie is concerned... Obviously Heaven's Feel will spoil plot points of UBW so if you're thinking about watching UBW at any point, you might as well do it before Heaven's Feel...
I'd also add that I would like Kajiura to explore "with or against" (and its type of BGM) in the future movies. Really made me think of Prince of Persia Sands of Time trilogy. Mufufu.
About your CGI fest, I agree with that. Especially when Lancer is fighting with Assassin on the truck then everything goes BOOM! Their 2D & 3D combination doesn't work well with that scene at all and some scenes too.