I don't think there are any subjunctives here.
Habitual "be" or invariant "be" is associated particularly with, but not exclusively with, African-American English - see https://ell.stackexchange.com/a/180966/60894 and https://ygdp.yale.edu/phenomena/invariant-be
Habitual "be" is also often seen in memes, especially in the phrase "be like" - as parodied here: https://memegenerator.net/instance/85168069/willy-wonka-white-people-using-habitual-be-in-memes-be-like-be-like
In the case of the podcaster that you quote:
'You just be bored during the pandemic.'
This could easily be a use of habitual "be". I think this should be distinguished from the subjunctive.
We should note that habitual "be" isn't considered part of "standard English" and isn't taught in EFL/ESL.
But what about your first example?
'...But I didn't realize that feeling so confident and feeling so great about myself ... and then it just be completely shattered by one thing.'
This could be another dialectal usage, but I am leaning toward your suggestion that it is a syncopated form of an expression that uses "be"-infinitive, such as "and then for it to just be" or "and then to have it just be". This abbreviated form of expression certainly wouldn't be accepted in careful speech or writing, though.
You say, "I would like to think this use of be is an intentional choice rather than a grammatical error given how the opening monologue is such a brilliant performance." But I think that writers, poets, singers, performers will sometimes use colloquial and non-standard expressions or play fast and loose with the formal rules of grammar. This may be a conscious artistic choice.
The subjunctive (sometimes called the "present" subjunctive to distinguish it from the irrealis "were" form) is identical to the base form or plain form of the verb (which in turn is identical to the bare infinitive and to the imperative, and for all verbs other than "be" it's identical to every present-tense form except for the third-person singular).
The best known uses of the subjunctive are (a) in set-phrases like "be that as it may", "so be it", "long live the Queen!" and (b) in mandative expressions in subordinate clauses following verbs of ordering or suggesting, e.g. "she demanded that he leave".
It is occasionally found in conditionals, but this usage is increasingly archaic, and is not recommended to learners: "be they dead or alive", "whether it be red or blue".
English is well known for the syncretism of its verbal conjugations. That is to say, the fact that often one form servers multiple purposes (infinitive, imperative, subjunctive, etc). It should not be assumed that the label "subjunctive" is appropriate for invariant "be" just because it shares a form with the subjunctive. Indeed, in Dorset dialect "be" routinely replaces "am".