You are talking about some pretty extreme forces that you likely don't yet understand. Not saying that I do mind ya, only up to a J myself.
It's hard to have a physical grasp of the forces until you launch something in HPR. Even for engineering students doing math word problems about forces nearly daily. You can stack an H onto a I in multistage and touch 22,000ft in a sim and in reality you can bust the sound barrier in boost mode of booster 0.3s. Seconds after launch then implode an interstage between the sustainer and booster on a MD CF rocket. Conservatively speaking the kinetic energy of the SEDS competition rocket for attempting record altitude for specific impulse was exceeding a light 37mm AA flak gun at the muzzle comparing to max sustainer mach and mass. A university research center predicted the nosecone would implode. They were wrong. And we were even more wrong as students. The CF tubing sheared in half due to sanding it down fingernail thin from zero experience at this and not having a true earned respect for forces involved. We had a 340 lbf thrust booster motor acting on a 2 lbm rocket. It was a bloody miracle it made it to 2,500ft flew straight, then interstage implosion from wobbly connection. The whole airframe simply shredded above a case bonded casing to airframe tubing section. Oddly the fins well some of them were somehow still attached despite being airfoiled and too thin. Shoulder cracked at nosecone, oddly nosecone survived, electronics fell to earth no chute and survived the fall with batteries disconnected. Could not turn head fast enough vertically to track acceleration off pad. 168G's pulled on a whimpy "pud knocker motor" as this 14 year old thinks ( not trying to insult, he just hasn't tried one and seen it first hand), 38mm I1299N-P Aerotech just a lowly L-1 motor... It was touching a K class thrust curve for that puny third of a second.
No one on TRF was posting MD multistages with it. Shared the same fuel composition as Orbital Peagsus with a fast burn time. Triple the thrust curve of the other I motors for class. Saw one thread disappearing act, how it ripped apart another single stage rocket no recovery or something from G forces involved. We found out hard way why. We lost tracking with FCC GPS tracker with a group of satellites. Not even a full I. The other was a CTI 29mm H motor which didn't light from robust raven logic. We'd spent months and thousands of dollars not having experience to get that far to make UROC launch date from nothing. It wasn't a kit you could go buy anywhere. It was our first design and clearly we were over our heads experience wise. Not treading water. Learning by Drowning. We didn't have mentors and reality was a hard teacher on forces in forced involved on HPR materials.
Halved the altitude expected, went to smaller airframe diameter, used two H motors, built something simple for a fifth the cost, and it worked well enough to meet mission requirements by competition on fiberglass airframe not sanded on a single stage then went third nationally. The old geezers in Utah had told us to our faces do not maximize thrust on rockets anymore, you want maximum burn time for altitude. We did not know that previously. I maximized the specific impulse as close to 640Ns with a maximized thrust on the lightest smallest diameter and most aerodynamic design I could do and that was a very poor decision for zero experience. We never had get there itis. It was a poor mistake from zero experience in HPR. First HPR I had designed and it had to be a multistage for the senior design class/comp, bad combo. It sure wasn't like any Estes low power flight I'd previously done as a child. Now the second iterations of nosecones worked as designed, but a commercial reload exploded from wiring lack of experience head end ignition.
Now David is going to tell me to go take my meds... I thought my personal story of screwup may someday save this 14 year old kid a moment from dreams being crushed by physics, budgets, and reality. Not what happens on paper, but what real world can throw your way when your wrong. You gotta get back up and try again. Just because you can computer model then manufacture a NACA 65A supersonic airfoil on a hobby rocket doesn't mean you should. Just because you can stuff a 75mm motor into a DX3 doesn't make it wise. And you can implode HPR's into tiny little carbon fiber fragments on a lot less thrust than a 75mm.
The closer to pushing a altitude record the more exponentially wrong things can go badly and you won't get your rocket back. Purdue also shredded a 20,000ft SEDS rocket this year named Proton or whatever...
Unlike Purdue, We had the guts to launch something lower performance a second time around at own expense and in a very humbled attempt to learn from our mistakes. Only 20 of 88 teams actually made it past launch phase into HPR multistages. Because the competition asks complete noobs to "just try it bro" and it's just hard enough to get a basic design subsonic to work on smaller L-1 motors and get past paperwork. And engineering students are naturally slightly arrogant thinking we think this will work the math theory says this. Then reality just slaps them real hard because its not always a cute theory.
Heard a story of $100,000 liquid rocket Vulcan 1 at UCSD had crashed because one altimeter wire disconnected. Heard another story of another team's liquid rocket having fin stabilized canards and ripping itself to pieces at 8,000ft. The engineering students didn't know the bending moments exactly in reality that were to occur during flight on the angled fins. They knew there were forces in component directions.
We had met L-3 guys and gals at UROC with rockets longer than our vehicles, airframes imploded, and they had more experience than us. Complete aluminum and fiberglass glassed shattered airframes and casings dented from forces involved. Even saw a huge sugar rocket explode at Hellfire 22 first hand. The forces involved threw a flaming sugar grain possibly 50-60ft into air from the pad, and popped the nose off. It was 12ft tall. The forces involved are mind blowing and the paper values do not reflect the true consequences of mistakes seen in person.
Start with the 29 or 38mm H or I because the forces only get greater. Still had to sign a death release and insurance health form waiver to do anything for college competition. Thankfully on your own you don't need to sign that form. Everyone that has pushed the limits has a shredded, exploded, or imploded rocket to show for lessons learned. Lessons that no book will teach you.