You cannot get below 0.25% from the code to avoid these cheesy strats, and even with Florentine ideas, economic and forgive usury, I don't see how you can go below 1 without turning Hindu, which is far from easy and would block you from getting "forgive usury" ever again. Maybe special events, but they are unreliable.
You will need to pay these loans, at a rate of +50 gold each time if you sit at 0.25, or at the more reasonable rate of +200 ducats each time with 1% interest. If you think taking 20k worth of loan might make you win 200 ducats in the next 5 years, and actually more because of inflation, go for it. This might be an interesting strat. Now you have the exact numbers. But this requires you to always have forgive usury on, or to get Hinduism, else the cost is doubled to repay.
Now imagine you use some of this gold to go over FL, you need to win back the money you spent this way, so you can't spend the likes of several thousands.
Overall the interest rates are pretty low but this is not that different from any normal loan heavy strat, there is a mere 1-2% difference from having more standard interest. By going this cheesy road, you can take 200 of loans to attack a neighbour, and you just save 4 ducats or so as compared to someone who doesn't have these bonuses. This doesn't really make a difference imo.