Hey BJ, you forgot the nightmare scenario of Tatum not signing with anybody next summer and instead playing out his last year on his QO; therefore giving him the option to go wherever he pleases in 2022. Hey, at least we get him for 2 more years 
That's actually option number 2.
When a player becomes a restricted free agent, you can offer them 1 of 2 qualifying offers: a normal one, which is for one year, or the max qualifying offer, which is 5 years with no options. We'll offer the max one, so the worst case is actually him signing a 3 or 4 year offer sheet with someone else, which we would then match (or, I guess retiring from the NBA to avoid playing for us, but we would hold his rights indefinitely so he would have to quit the NBA forever/until we traded his rights).
There's no way we can lose him before July 2024, unless we want to
Edit:
See question #42 on saltlover's Larry Coon's CBA FAQ