Looks like this will help as long as the delay isn't too long. I stopped building 4 inch diameter cardboard rockets due to this issue. Yeah, I'd fiberglass some of the 4 inch tubes but a late delay can lead to a zipper no matter what using motor delay that's mismatched. Much easier to use heavier fiberglass tubing. Heck, in the Midwest can't fly that high anyways! For small projects as shown, I think this technique will work quite well though and will consider adding it to some of my small projects. The deal is, is how much mass is in the nosecone or if there is one, a payload bay on the end of the harness? More mass, more damage on a late deployment. That's just physics for you.
I have a tendency to sim like crazy with Rocksim and Open Rocket to chose a delay grain (or drill it) if I need a shorter delay. I've had to order extra "long delay" grains from Aerotech or my local dealer and drill them to what I needed for a delay time based on the simulations with the larger impulse motors. Always gave me a good apogee deployment at minimal velocity (ideal) on apogee only rockets.
It's a moot point with electronics 'cause when the rocket tips over at apogee, "pop goes the main parachute or drogue". I "sorta" switched to electronics to relieve the stress of getting motor deploy "just right" on high powered rockets. Modrocs not so critical. I still like a challenge and have a few rockets I need to do preflight calculations on with sims using motor only deployment. Shoot, can have the motor deploy at apogee and no zippers.
I had some rockets that flew with the Robert Galejs magnetic anomaly detector for apogee deployment: https://aeroconsystems.com/cart/flight-avionics/magnetic-apogee-detector/
which is out of production now and added a Jolly Logic chute release later on when it became available. I now had dual deploy in my MAD originally apogee only rockets! I later bought a Zeptomag device: https://www.tindie.com/products/ZeptoBit/zeptomag/
but it seems both are out of production now. It has more safety to it as if armed for launch, if one has to replace the igniter for a misfire, one can turn it over without turning it off and not have it go off. A G-Switch fully arms the device just after launch in flight. I always turned it off when I rarely had to take a rocket off the pad. The Galejs device, one had to turn it off and gingerly remove the nosecone upright and unplug one leg of the ematch wire from the terminal to avoid a ground fire of the deployment charge.
I'm surprised there isn't a small MAD unit for smaller rockets now. When the Rocket Logic Chute Deploy came out, I got dual deploy on my smaller MAD apogee only rockets. Because of this, I'd launch with bigger motors with the longest smoke delay I could get. Many times the rocket would be way out of sight but I could see a descending smoke trail for awhile after the unseen apogee and knew the chute bag deployed. The chute release would unfurl the main at the selected lower altitude and at least I had a direction to start at as I saw the smoke trail of the descending rocket up high.
When the smaller dual deploy altimeters came out, this combo MAD and JLR chute release sort of became a moot point (unless one wanted to save on ematches!) but it's in a few of my flyable rockets and I'll continue to use it as I have the experience to make it work every time. A single deploy, apogee only altimeter and a JL chute release is viable too as long as the body tube diameter is enough to contain the JLCR. Kurt