Estimated taxes are paid by the individual, not the business (LLC). And whatever interest the business earns is considered taxable, which thus subjects it to FI

Estimated taxes are paid by the individual, not the business (LLC). And whatever interest the business earns is considered taxable, which thus subjects it to FICA, i.e. business interest is effectively earned income; whereas if it's individual interest it's unearned income and FICA does not apply. You should discuss this with a CPA. Accumulate all of your questions, schedule an hour, email them in advance, go in person to get them answered and discuss.