Can you elaborate on why you don't want to be transonic during staging? I have seen lots of advice about not spending a lot of time transonic, but are there other issues specific to staging?
Why do something complicated requiring smooth, perturbation-free release of a sustainer during the period of highest instability, vibration and aerodynamic stress?
In addition to this, your Raven 3 programming gets harder as the rapid pressure drops could fool an altitude versus time logic. It'll specify a velocity versus time with pressure trigger logic or something specific for transonic. In addition at Mach 1 there are oblique shockwaves (Conical AIR version of concrete speedbumps for all sharp objects) in CFD that are visible at Mach 1 with rapid pressure changes. In addition before/after supersonic you have transonic flow causing turbulent flow boundary layer problems. Shockwaves can reflect. If airfoiled you can expect airfoil performance to rapidly change.
I remember from advanced compressible gas dynamics that the shock goes from supersonic to subsonic for normal shocks. But oblique shocks were related to the angle of object which is theoretical as a half wedge. Longer cones were better for that theoretically with lower angles.
In addition, "If a reducing diameter stage rocket design doesn't drag separate by maximum booster velocity then it will not drag separate", Dr. Sreenivas UTC Sim Center, the guy that did our comp rocket CFD. Now you have a burned out booster stuck to a sustainer stage with an unexpected unwanted CG shift massive aft of initial design expectations because you tried separation at transonic and the drag force depending on velocity wasn't high enough for drag separate.
Only designed and flown two L-1 hpr multistages for USRC SEDS 2017 comps, got 3rd. Avoid transonic like the plague. Avoid real spikey thrust curved motors on lightweight rockets with thrust specs exceeding class of motors your limited too. i1299N-P destroys interstages/disconnects batteries, lol.
Keep the design if its MD screamer and supersonic is unavoidable, completely supersonic stage separation and ignition (this is risky). I've tried a M1.5 stage separate/ignite, we destroyed a interstage. Structural has got to be top notch or its not gonna survive. You also don't want the fins to flutter and your sustainer could easily wind up around M2.4 or greater. So thermally your epoxies get expensive. So then the upper limit will be the fin design and flutter predictions on fins if supersonic stage separation/ignition is even viable or not.
Or slow it down. Subsonic booster stage separate, subsonic sustainer ignition, and let sustainer stage itself no booster attached accelerate under it's own power through trans and supersonic regions. Transonic is not a region you want to play around with multistage rockets. There's many ways to screw up a multistage, more than you think. Keep in mind the drag force depends on velocity squared. This was the second route I've tried.
Don't maximize thrust for specific impulse of class is what I learned first try about multistage motor selection on lightweight rockets.
Back to op, triangle thrust curves with high peak thrust at very steep slopes are bad for light multistage rockets, they will sim high, with stupid amounts of acceleration in excess of trackers or neck turning ability. Any mild thrust curve booster with a long burn sustainer will beat that.
Jim Jarvis has a youtube vid up of a sustainer failed ignition with sustainer laterally ripping itself off of a "good" interstage then flying vertically past a booster still burning. You see it from the rocket's point of view. Transonic and supersonic multistages shouldn't be taken lightly. Some real world behaviors are beyond CFD capabilities to simulate.
When it's really unstable in transonic you could change the fin angle of attack severely at high Mach number beyond fin joint limits and destroy fin by bending stresses or alter rocket course off planned path if it poorly separates at a bad angle.
Sorry for the long rant...