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Not LEO but just above the Karman line

The alternative, actual LEO (orbit):

I don't think anyone is going to be deploying cubesats at 100 km, MECO (main engine cutoff) is below that, looks like roughly 70 km and you'll be passing Karman during the middle of 2nd stage burn, not a time when anyone will want to be deploying cubesats.

The lowest altitude regular deployments would be the Starlinks at roughly 250 km, you can check if they do concurrent cubesat deployments.

Certainly there are from time to time missions launched at very low altitude, and some might have concurrent secondary payload programs for cubesats.

Atomic oxygen and space weathering (erosion) - beware!

note: Become very familiar with the effects of atomic oxygen (O rather than O2) on the materials on the outside and the inside of your spacecraft!

Msis atmospheric composition by height https://commons.wikimedia.org/wiki/File:Msis_atmospheric_composition_by_height.svg source click for larger

Also related to VLEO (Very Low Earth Orbit):

The alternative

Not LEO but just above the Karman line

The alternative, actual LEO (orbit):

I don't think anyone is going to be deploying cubesats at 100 km, MECO (main engine cutoff) is below that, looks like roughly 70 km and you'll be passing Karman during the middle of 2nd stage burn, not a time when anyone will want to be deploying cubesats.

The lowest altitude regular deployments would be the Starlinks at roughly 250 km, you can check if they do concurrent cubesat deployments.

Certainly there are from time to time missions launched at very low altitude, and some might have concurrent secondary payload programs for cubesats.

Atomic oxygen and space weathering (erosion) - beware!

note: Become very familiar with the effects of atomic oxygen (O rather than O2) on the materials on the outside and the inside of your spacecraft!

Msis atmospheric composition by height https://commons.wikimedia.org/wiki/File:Msis_atmospheric_composition_by_height.svg source click for larger

Also related to VLEO (Very Low Earth Orbit):

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What you'd like to might turn out to be much harder than getting a cubesat into orbit, unless you can find a sounding rocket launch provider that is interested in working with you.

These days the process for getting cubesats to orbit is not easy, but it's more established and standardized and predictable than it was in the early days. It still costs money and takes time and investment and clearance and proposals... but the trail has been blazed and you'll be on a well-worn path.

I think it is extremely interesting to pursue something different! Suborbital cubesats are a whole new thing with new opportunities and new problems.

For example you can't guarantee burn up on reentry, but the trajectory of the sounding rocket that takes you up will likely put you in a situation where you can return to earth without hitting people.

Now if your cubesat pops out some kind of lift device and you want to be a "Karman plane" (What would a "Kármán plane" look like, a bird, or a plane?) that's a whole different story - you'll run in to a lot of regulatory issues. But if you just want to fall back to Earth along with the rocket body, perhaps even tethered to it, maybe this part will not be a regulatory challenge.

You will have very different power management and attitude control issues than an orbital cubesat, which means you will have fun adapting cubesat COTS stuff or designing your own. Since life is guaranteed to be short maybe just a battery? Cubesats to orbit have stored energy regulations and they like you to deploy without fully charged batteries and rely on your solar panels to charge in orbit to wake up (at least that used to be true). Sounding rockets on the other hand are absolutely compatible with launching with fully charged batteries.

Communications may be easier - many (most? all?) sounding rockets rely on real time telemetry - they send all their data to the ground in real time so that if the payload isn't recovered (pretty common for launches over water) the mission is still a success. If you remain tethered to the souding rocket there may (or may not) be a way to share the same downlink station. But those technical details you'll have to work out with the launch provider. You're guaranteed an antenna pointing at your cubesat, but maybe your tether will be a USB cable.

This sounds new and exciting and challenging. Step 1, look for education outreach programs from sounding rocket launch providers and get to know some folks and see what they think.

The alternative

Do a normal launch to orbit, deploy your reentry device (like a drag chute, there are several styles for cubesats) immediately and just hibernate until you reach the Karman line, then do your thing quickly as you rapdily heat up and disintegrate.