That's certainly an issue, but from a business perspective, if the assets are worth a lot more, then the banks' exposure is zero/minimal in the event of a default.
Also, not all foreign lenders are evil-doers. The divide between respectable versus not respectable isn't at the US border. If Western European or Japanese lenders are providing funding against local Trump golf courses, I don't see a problem. Those would appear in his US accounts.
He has a portfolio of assets all over the world. Many of them are valuable. So the existence of $421m in debts is not alarming on its own. It's the balance between assets and loans that matters.
If he is borrowing from Saudi royalty or Putin, that's different. But I don't think the mere existence of loans provides leverage unless he is dependent on dodgy providers for refinancing.
As regards his net income, I think we can already see from the kinds of expenses he is claiming that his net income, properly calculated, is substantially positive. He's hiding his actual net income behind things like fake consultancy fees to family members. It's his daughter (and her husband) that seems to have useful losses to set Trump's income against.
Family tax racket. I think that's where his criminal exposure is going to be.
The $750 tax payment may doom him politically. But I think it's not really representative of his net income and the state of his business. It's an engineered number.