After two weeks of reading about liens and deeds I'm worse at this

A tax lien certificate is a claim for the unpaid taxes. It isn't ownership of the property. You pay the county what the owner owes, the county issues you a certificate, and if the owner pays up during the redemption period you get your money back plus interest at the rate set by state law. If nobody redeems, you can begin a separate legal process to foreclose the lien, which has its own notice requirements and deadlines and normally means hiring an attorney.

A tax deed sale works differently. The county sells the property itself and the winning bidder ends up with a deed. Some states sell liens, some sell deeds, and a few sell a deed that the former owner can still redeem for a set period, so the first thing to pin down is what your target county actually sells.

The 16% and 18% figures are statutory ceilings in certain states. In a busy auction, bidders compete by bidding the rate down or by paying a premium over the taxes owed, so the yield you actually get is usually well under the headline. Most certificates redeem, so build the plan around collecting interest and treat a property landing in your lap as a rare event.

Entry is cheap next to buying a house. Individual certificates can run a few hundred dollars, though counties want a bidder registration and often a deposit before you bid. What catches people out is subsequent taxes. In many states, holding the certificate means you also need to keep paying each new year's tax bill to protect your position, so money keeps leaving your account after the auction is over.

Florida is the one I keep circling on my list. The certificates on anything halfway decent clear at 0.25%, which looks pointless until you find the minimum interest rule in the statute, a 5% floor on redemption unless you bid zero. So a 0.25% bid that redeems in three months is a fine annualized number and one that sits for two years is a bad one. Same bid, completely different outcome, and you don't get to pick. I have 40 parcels in a spreadsheet and no bidder number yet.

The thing that stopped me cold on the deed side was payment timing. Some counties want the full balance wired by the next morning, no financing, no inspection, no walkthrough.