You should draft a QB early. If in round 1 depends on your draft position. If you have a late spot, and the top 5-6 QBs are gone already, I’d recommend to take a top RB or WR here.
If you pick early in round 1, or if few teams have picked a QB, I’d indeed recommend to draft a QB first, and maybe even second as well, as @edmcgon suggested.
And if you go for a non-QB first, I’d still recommend to not wait too long. Latest by round 5, you should have 2 QBs on your roster. A “zero QB” strategy absolutely does not work in SFLEX leagues
I’d also recommend to go after a 3rd QB around round 8-9. Keep in mind that, in a 12-team league, not all teams can get a 3rd QB. But you will need one for bye weeks or in case of injuries.
The reason QBs are so important in SFLEX league is because they score so many more points than RBs or WRs. In half-PPR, the average QB#25 scores as many points per game as the RB#5 or the WR#3. If you have to start a 2nd tier RB or WR on SFLEX, you will easily lose 7-8 points compared to a 2nd tier QB.
The fall-off characteristic of the QB curve is also a little different from the RB or WR curve. The QB position also sees a pretty steep drop-off after the elite tier. But while the RB and WR curve flattens after that post-elite dropoff, the QB curve keeps falling.
The difference between the #10 and #20 player on RB and WR is around 2 FFP per game.
The difference between the #10 and #20 QB is 4 FFP per game.
In other words: if you don’t get a top 10 RB or WR, you can afford to skip that position for a round, as it won’t make a huge difference if you get the RB#12 or the RB#18 in the end.
It does make a difference if you get the QB#12 or QB#18, though.