For now the quick reference grammar sounds sufficient. However, I would add the basic relative pronouns because they appear everywhere.
Vide:
Singular
M F N
Nom. qui quae quod
Gen. cuius cuius cuius
Dat. cui cui cui
Acc. quem quam quod
Abl. quo qua quo
Plural
Nom. qui quae quae
Gen. quorum quarum quorum
Dat. quibus quibus quibus
Acc. quos quas quae
Abl. quibus quibus quibus
Want to learn the interrogative pronouns, too? Well, now all you need to know is that the Masculine Singular ‘qui’ changes to ‘quis’, and in the accusative neuter singular you use ‘quid’ instead of ‘quod’!
Avoid the early pitfall of trying to memorize everything with equal emphasis. The full noun declensions and verb conjugations are by far the most important. Stuff like different kinds of adjectives are easy and much less important. Besides, the exceptions are often only slightly different and you’ll be able to quickly integrate them into your understanding once you know the basics. I don’t know what the frequency chart would look like, but you won’t be using even all the declensions or conjugations with equal frequency. Most verbs will be indicative, most nouns will be the first three declensions, and you’ll deal with a fairly regular set of constructions. Think of English: you don’t run through the entire grammatical gamut every time you write or speak, but use a basic and repeated set of grammar and vocabulary. How often does one use the subjunctive imperfect passive second person plural (laudo-> laudaremini)? Rarely.
For example, if you’ve figured out your noun declensions, then you’ve covered most adjectives, which decline in the exact same ways. You’ll encounter adjectives with one (‘ferox’) and two-endings (‘subtilis’). But if you’ve learned your declensions, then these show only minor differences (often in the nominative) that you’ll figure out without having to memorize (e.g. write out 200 times) a separate declension.
A language like Latin appears incredibly intimidating when you’re starting out because there seem to be thousands of disconnected and arbitrary facts that you have to simultaneously keep in your head. In reality, there is a core set of basic structures and processes (a sort of grammatical skeleton) that organize the whole language. You’ll find that the more you learn, the easier it will become. The beginning is by far the hardest, and the rest is details. Once you’ve cleared the “quick reference grammar” you’ll be able to advance with leaps and bounds.
I’d think of the memorization as a supplement to Lingua Latina, and not the other way around. It’ll give you a background for the grammar you’re seeing in LL, but you won’t really understand it until you’ve done it in context.
Also, if you find that you can’t stomach all the memorization, take a break and just read LL without thinking about the grammar.