As James K said, this doesn't have anything to do with the spelling.
Different accents of English have different patterns for the use of "dark l" vs. "light l". As you mentioned, some accents are even described as having only one of these sounds. Accents with "both" sounds are often described as actually having a "gradient" of "darker" vs. "lighter" /l/s in different environments. So this is a pretty complicated question.
In general, there seems to be a tendency for "dark l" sounds to be used more often in American accents than in British accents.
In the British "Received Pronunciation" accent of English that is often taught to learners, "light l" is used before a vowel sound, even when the /l/ comes at the end of a word or after a stressed vowel. This accent would have "light l" in "killer", "bellicose", and "pullup". (I don't know if the "light l" in "pullup" might be a bit darker than the light l used in other contexts; that sounds somewhat plausible to me.)
In certain American English accents, "dark l" is used before consonants, and also word-finally, even when a vowel follows, but "light l" is used before a vowel in most word-medial contexts. These accents would have "light l" in "bellicose", but because "pullup" is a compound, I think it might have dark l. I'm also not sure how words like ""killer" are treated in these accents.
In other American English accents (like mine), it seems that "dark l" is used whenever the /l/ is not at the start of a metrical "foot" (either as the very first sound, as in "lucky", or as an element of the onset cluster in the first syllable of the foot, as in "climbing"). So I have "dark l" in killer, bellicose and pullup because the /l/ is not at the start of a foot, but I have "light l" in words like illiterate, illuminate, crystallography, collect where the /l/ is at the start of a stressed syllable.
I quoted some sources in my answer to a related ELU question: L in the middle of a word: dark l or light l?
Best Answer
Some speakers will actually sound "you" more like "ya" when speaking quickly
This is dialect and probably shouldn't be imitated too closely (unless you want to sound like you're from New York / New Jersey), but otherwise you are correct and when "you" is part of a phrase the vowel is normally short, and often combined with the following word
Pronouncing "you" with a "j" sound is, again, dialect, common among people who also speak Spanish, where the "y" sound is pronounced like the English "j". Again, it's not something you should imitate unless you are trying to sound like someone who speaks that dialect.
There is an in-between sound that is more like "choo", particularly when "you" comes after a word that ends in "t", like "what".
Most speakers use this pronunciation to one degree or another, unless they are intentionally careful with their diction.