And it's not always the case that tax cuts mostly benefit the wealthy. The yardstick matters. In absolute dollars; yes, usually the wealthy benefit more because their incomes are greater. As percentages of disposable income, lower classes sometimes do better. And obviously people paying little or no tax get little or no benefit.
I cannot emphasize enough : direct spending is only one use; I've noted others above, all beneficial to growing economies and putting more income in the hands of workers.
Depends on what you accept for comparison.
Here is at least a proxy, if not exactly the quantity you have in mind. Current rates are somewhere in the mid range between historical highs and lows. And since the cost is basically rate * principal and the principal amount is very high right now, arguably the US "cannot afford". There's a mix of opinions as to whether the debt is unmanageable yet or nearly so.
Most "investments" governments make are really just consumption spending.