Your mortgage is costing you 7%, that gives you a 4% cushion. The 7% is the amount you have to pay regardless of how STRC is doing. If STRC lowers the dividend to 7% because of high demands, you lose your cushion. If STRC dumps 20%, you lose your capital. If Saylor dies, your probably lose 50% of the invested capital. You are talking a paid off home (zero risk) collateralized it with a Bitcoin derivative (high risk). Too much risk to take for a 4% spread.
There is no free lunch.
Stay humble and stack sats.