"Also, keep in mind that shastras don't mention the duties of mlecchas. The shastras are strictly for Indians. - yes. no point in enacting laws for dogs and cats. they just do what they please and face the consequences. similarly mlecchas have different religions (christianity, islam) to cater to their lowered morality.

"Nama sankirtana" is not going to magically grow crops for you." - well, it might, but that wasn't Bhagavan's intention in making one chant. and I didn't say it will grow crops.

"So then no one should take up farming, business, military," - you are attacking some wild strawmen here. they can take up farming. and they will take it up. AND they will face the consequences of it too. Like i said earlier, just by being born in Kali, you are doomed to face pain. It's like hospice care. Shastras knows people will commit sin and suffer, so it gives them a pain-killer (nama sankirtan).

"if no one can take up a job if that caste doesn't exist" - are you looking for a rule in shastras that justifies any work done today? and if it doesn't exist, you assume shastras are outdated ? i'm saying shastras know about all types of work that will be done in future. and the point of shastras is to get people to follow rules to reach Bhagavan. and Shastras know that these people doing random work will not follow varna-ashrama rules. So it doesn't mention these random works/technologies

Instead they give the only way out which will work in Kali Yug - nama sankirtan.

also like i said earlier, lot of mixed caste professions are already mentioned. all modern jobs fall under one of these buckets.