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.