Don't take financial advice from broke cousins.
Don't let them kill your dreams, because they don't have any.
I would go to far as to say, don't ever talk finances with that cousin again. Don't tell him of your plans, don't tell him about your investments, don't ask him about his. Not the right person to support you.
Disadvantages1-2-3-4-5-6-7 all true and possible, this smells like opportunity, probably more risks than that....and property is more work than T-bills.
Advantages....1 is possible....not sure I understand 2....no cash flow from your side if you have vacancy. 1031 defers taxes not eliminate it, but there are ways perhaps, for example if you get big enough and 1031 into QOZ you have the poential to eliminate 100% of the capitol gains taxes.
#4 you forgot about leverage....for the most part you can't leverage T-bills, maybe 50% on ETFs. ETFs can fall to zero...or even lose 20-30% in a day. People can and do lose money in ETFs, go back and find the paragraph about the average investor loss in Fidelity Magellen fund back during Peter Lynch tenure. I forgot the numbers, but fund went up like crazy, but avg investor tried to time the market and lost money. Buying at the highs, selling at the lows.
Ask relators and investors in the Abilene area if prices have gone up or down in 2023-2024 or if they wish they had more money to buy more homes in the 1970s, 1980s, 1990s, 2000s, or whatever decade. I've never met a long term real estate investor who wished they had bought less property.
If you invest $100,000 in T-bills this year at 5% in a year your return is 5000.
If you invest $20,000 to buy $100,000 of real estate and it goes up 5% your return on the $20,000 is 25%, not including the rent, the depreciation, the tax advantage, the ability to defer taxes, and your tenant paid down $1000-$1500 of your mortgage....these are the bare basics.