@Jesse Munos
I don't have experience with tax liens, however, you may look into promissory notes via trust deed. I sold my SFR rentals making around $1k a month and rolled the funds (trapped equity) in a note backed by real estate, instead of buying a bigger rental (can't sell high and buy low very easily), and now netting $60k a year. The collateral is an RV park in Vancouver WA. I have more details on that in my profile along with several smaller note I've done. I also have notes in my self-directed ROTH and IRA making a minimum of 15% a year.
The due diligence mainly focuses on assigning a conservative value on the collateral and doing a rough estimate of your investment coverage, e.g. ARV of $500k - $200k (1st position) - $60k (2nd position - your note) = $240k (post-foreclosure potential surplus). We don't plan to foreclosure but that is our legal means of security. In this simple example, I would suspect you would sleep well knowing you were making 8-15% and your principal is covered by over 4 times your principal. I do this locally so I am more familiar with market trends and collateral values, however, doing remotely you could hire local experts to weigh in. I use a local real estate agent and a title company to confirm project assumptions and takes a few hours for me at this point.
A common question I receive is "why would a flipper use such expensive hard money?" The answer requires to switch rolls with the flipper. Say you are starting 1-2 flips a month. At some point, you run low on funds/credit. For me, that would be month 1, others may take years. Now if you were in a position to buy another property but your funds/credit was tied up, would you pass on the deal? Wholesale it? Or use other people's money? The acquisition requires fast and reliable funding to seal the deal in all markets at this point in the mania. This is where a reliable capital partner with the direct means to fund a note within a few days is more than worth a higher rate versus leaving profits on the table. Conventional or more institutional HML have too much internal red-tape and too many levels of approval. Not saying these sources aren't used, however, adding a capital partner to the mix greatly improves business stability.