Formal alternative to bullsh-t

"Nonsense" would be my first choice too. "Absurd nonsense" is even stronger.

"Nonsensical" might be a good choice too.

Nonsensical: If you say that something is nonsensical, you think it is stupid, ridiculous, or untrue — Collins Dictionary

I see two useful approaches here. First, something like "Everything he says is unfounded," or "His whole essay is incoherent." These two suggest Bob is making a weak, ineffective argument. From Collins Dictionary:

If you describe a rumour, belief, or feeling as unfounded, you mean that it is wrong and is not based on facts or evidence.

If someone is incoherent, they are talking in a confused and unclear way.

If you say that something such as a policy is incoherent, you are criticizing it because the different parts of it do not fit together properly.

Incoherent - Collins Dictionary

Unfounded - Collins Dictionary

If you want something a little more directly critical but still formally phrased, you could call his bullshit "empty rhetoric." This suggests he's deliberately using linguistic tricks to obscure a lack of content. That's appropriate for a formal register, but it does come across more like accusing Bob of deliberately bullshitting, rather than simply failing to make a good argument.

Collins again, 'rhetoric' definition sense 4:

4. speech or discourse that pretends to significance but lacks true meaning

Rhetoric - Collins Dictionary

There is "humbug"... which at least in writings of the 1980s seemed to be granted consideration as a word in current use.

Harry G. Frankfurt's essay / book "On Bullshit" has this to say - I think he came pretty close to a "working" definition by relating "bullshit" to "humbug":

I am uncertain just how close in meaning the word humbug is to the word bullshit. Of course the words are not freely and fully interchangeable; it is clear that they are used differently. But the difference appears on the whole to have more to do with considerations of gentility... It is more polite, as well as less intense, to say "Humbug!" than to say "Bullshit!" ... I shall assume that there is no other important difference between the two.

(bold emphasis added)

and

Black suggests a number of synonyms for humbug, including the following: balderdash, claptrap, hokum, drivel, buncombe, imposture, and quackery. ... [He] also confronts the problem of establishing the nature of humbug more directly, and he offers the following formal definition:

HUMBUG: deceptive misrepresentation, short of lying, especially by pretentious word or deed, of somebody's own thoughts, feelings, or attitudes.

A very similar formulation might plausibly be offered as enunciating the essential characteristics of bullshit.

By the last statement, IIRC, the author basically equates the terms in what seemed to me to be a very clear & compelling way.

The name "Black" refers to the citation: Max Black, The Prevalence of Humbug, Ithaca, Cornell University Press, 1985.

While not exact, I would suggest disingenuous . It gets across the idea that that there is an intent to mislead while attempting to sound helpful and informative.

https://www.merriam-webster.com/dictionary/disingenuous

lacking in candor also : giving a false appearance of simple frankness