Excellent effort, Heart of A Lion! Two points I would add that have already been made by others:

(1) Things look a little simpler once you understand animacy/inanimacy rules for determining the accusative form. (After all, there are many instances where an animate and inanimate noun have nearly identical declensions, with the accusative form being the only significant difference.)

(2) Similarly, things get simpler the better you understand "hard" vs. "soft" consonants -- because, again a hard-stem noun and a soft-stem noun may have nearly identical declensions, apart from the fact that, say, the inst. pl. is spelled -ами instead of -ями.

I would also avoid trying to figure out "how many declensions are there in total?", and instead recognize that there are, perhaps, 3 or 4 or 5 "major declensions" (depending on how you define "declension"), plus a bunch of minor exceptions that apply to a limited category of nouns. For example, the word котёнок ("kitten") has a "weird" plural -- it's котята, and not котёнки as you might logically expect. Does this mean that котёнок represents a separate declension? Maybe, maybe not -- I mean, the singular forms are otherwise logical and follow the usual pattern for a masc. and animate noun. And once you get past the stem change (from котён- to котят-), the plural endings are also "otherwise logical." Then you've got singular neuter nouns ending in -мя, like время, "time." There's an argument for calling such nouns a separate declension -- but then again, there are only TEN such nouns in the entire modern language, and at least half of these ten are used rather rarely and aren't at all important for a beginner to know. And then you've got truly oddball words like путь ("way, path"). Arguably, it's an irregular word that doesn't represent a declension in itself -- and shouldn't necessarily get its own line in your table -- because no other words in the language follow the same pattern. You could say the same about мать (gen. sg. матери) and дочь (gen. sg. дочери), "mother" and "daughter." Both words are weird and irregular -- and you've absolutely gotta learn them, because they're so common -- but they don't represent a bigger declension pattern that you need to worry about -- they're odd variants on the basic "Feminine Type II" declension, or whatever you wanna call it.

P.S. I would probably drop the дитя row from your table. The plural form дети ("children") is in its own category as a must-know irregular declension, but the singular дитя is nowadays used pretty much only in the nominative, and mostly in poetic or proverbial contexts.