Is "there are so much better alternatives" grammatically correct?

A person used the phrase "there are so much better alternatives" when saying that there are significantly superior alternatives. This phrase is meant as an emphasized version of "there are much better alternatives." I don't know the formal rule, but the "so" seems to make the sentence sound off.

It's grammatical, but it's a little bit of a garden-path sentence (https://en.wikipedia.org/wiki/Garden-path_sentence).

As you acknowledge, "there are much better alternatives" sounds normal and is grammatical. It's a dummy subject, "there", followed by the plural cupola, "are", followed by an adverb, "much", that modifies an adjective, "better", that modifies the noun "ideas."

Also grammatical, and also typical, is the sentence, "there are ideas that are much better." Here, instead of preceding the noun, the adjective phrase "much better" comes after. But it's equally valid and has an identical meaning.

Then, we could add an additional adverb to the adjective phrase, to end up with "there are ideas that are so much better." Still very typical and grammatical.

Now, we should be able to move the adjective phrase back in front of the noun: "there are so much better ideas." This should still be grammatical for the same reason that "there are much better ideas" is grammatical (the extra adverb shouldn't change anything syntactically).

The problem is that hearing or reading "there are so..." primes us to expect the next word to be "many." So when we encounter the "much," we have to reanalyze the sentence.

If you were to slow down the process of parsing the sentence, it would be something like:

• "There" - OK, could be an adverb, could be a pronoun
• "are" - ah, so it's "there are," a pronoun
• "so" - adverb; the next word is either an adjective or an adverb; I bet it's "many"
• "much" - whoops, that's not "many" and it's an adverb; the speaker made a mistake
• "better" - huh? this should be the noun, that the adjective "many" is supposed to modify, but it's another adjective...
• "ideas" - "so much better ideas"; Oh! they did mean to use an adverb; guess I was wrong.

It's unpleasant to have to reanalyze a sentence (try those staple garden-path sentences: the horse raced passed the barn fell; the old man the boat; The complex houses married and single soldiers and their families). Better to just rephrase: "there are ideas that are so much better."