Look at it this way: even if you could borrow money at the risk-free rate (which you can't), anything you're earning on those leveraged funds is a risk premium (which by definition can't be diversified away). So you would certainly increase your odds of making more money, but you also increase the odds of a larger downturn, which again, could reach unsustainable levels.
I don't believe there exists a workaround to achieve a long-term (ie near indefinite) safe withdrawal rate of more than 3% or so in real dollars.
Bottom line is once you buy those bond funds you should only be spending your coupons/distributions. There's a chance your distributions might decrease if for example the yield curve flattens or interest rates decline and the fund has to roll over into lower yielding paper, but for the purposes of discussing FU money you should probably be buffered to withstand a moderate decrease in monthly interest payments anyway. Ie, in practice you're not going to spend your entire monthly distribution, so any cash leftover can be rolled back into your investments and likely moderate any of the risk incurred by not going with a full 'risk-free' portfolio.
That said, it's a lot easier to increase spending than to decrease it, so I'd want that risk to be small, or to have a desirable fallback for added income.
If you have no plans of ever liquidating, you can use leverage (and increase your exposure to interest rates, which is irrelevant if you will not liquidate) but get up to a 8-9% tax adjusted yield right now, so then your FU money drops to about 2.5-3M.