6 leagues last year, but usually play in 4-5 leagues where my teams are fairly streamlined. This past year was very difficult keeping up but I managed to win my first time ever in one league (workplace league) and my worst finish was 3rd.
This year I'm taking a step back and sticking to 2-3 so I can focus on making content for my main league that I'm the commissioner of.
It's something I started doing last year.
Right now the Lions and Chargers each have offensive lines where the average starter age is ~26 years old, and they each have multiple pro-bowlers and/or All-Pro/Rookie team members. Teams like that have long-term potential to keep a good cohesive unit together for a relatively cheap price.
Top WR talent will perform regardless of QB talent. Davante was still Top-5 despite Carr playing horribly.
Pay attention to OL quality when choosing a QB. Goff and Hurts had so much time to let plays develop and had great years as a result.
My main league is entering it's 5th year of IDP scoring, and I've been loving it and am probably the main proponent for it. Like you proposed, my league began with a single IDP Flex position and we steadily grew to now having a DL, LB, DB & IDP Flex positions on our team (We use Sleeper which doesn't differentiate between DE & DT, ILB & OLB, or CB & S).
I can't recall our initial scoring settings when it was a singular IDP flex position, however I recall that during both our first season with the single IDP & our second year with 4 IDP positions, we had to have emergency league meetings to change the scoring system because there were players racking up 30-50 points a week. Again, we use Sleeper, so some of these options may not be available for you. Currently our IDP Scoring format is as follows:
•IDP TD=6
•Sack=6
•Hit on QB=0.2
•Tackle=0
•Solo Tackle=1
•Assisted Tackle=0.5
•Tackle For Loss=1
•Blocked Punt, PAT or FG=5
•Interception=6
•Int Return Yards= 1 point every 10 yards
•Forced Fumble=3
•Fumble Recovery=(+0.10 per yard)
•Safety=6
•Pass Defended=3
•10+ Tackle Bonus=3
•2+ Sack Bonus=3
Looking at it now, there are a couple changes I'd like to make, but right now it's weighted to provide an equal opportunity to each of the 3 sections of the defense.