Yes, tax year 2018 is the base year for the 2020-2021 FAFSA.
Need-based aid is recalculated each year based on current assets and income from the prior-prior year.
Yes, it depends on the college. For most colleges, $300k annual income and non-retirement assets of $1 million will result in no need-based aid, even with two kids in college. The exceptions might be a few schools that have the most competitive admissions and are the most generous. Think Harvard, Princeton, Stanford, etc. Check out the Net Price Calculators that every school will have available on their website that will take an input of your particular financial and family situation and spit out the estimated net price that you will pay after scholarships and grants are applied.