For one, the vent holes could be a bit bigger. Adding a SHORT price of tube coupler also helps. Big thing I like to do is to apply thin CA, which while not burn-proof, does make cardboard and balsa more burn-resistant. I always add some thin CA to vent holes in tubes, and let it run inside of the tube a bit, but avoid letting it get into the engine area. If I use a full nose cone, then I coat the base of the nose cone with thick CA.
But the biggest trick I use with most Rocket Gliders, is to split a balsa nose cone in half lengthwise. So rather than side vent holes, the ejection charge blows out of the front, where the missing half of the nose is. Adds a bit of drag but not much and I like the tradeoff (been doing most burn-thread R/G's that way since the late 1970's). Also can use one nose cone for two gliders...... or have a spare half to use to replace the first nose half later as eventually it will get roasted.
Before gluing the nose half in, I first cut a shallow v-notch into the nose half, along the "base" area and the "flat side" where it was split, in part to make up a sort of ramp to guide some of the ejection charge that would be 50% blocked by the base (on some occasions I have gotten fancy, wrapped sandpaper around a suitable diameter of dowel, and sanded a curved "elliptical notch" into the nose half). But also, I have used a notch to give a bit of a hollow area for the burn thread to run across, so that hot gases fully engulf the thread. I have not documented that in a drawing, though.
In any case, I apply thick CA to the nose base, flat "half", and the V-notch, to inhibit burn damage. After gluing the nose half in, I cut out a 180 degree arc of tube coupler, the length of the nose base, and glue that to the area of the engine mount tube where the other half of the nose cone would have been, and apply a little bit of thin CA inside. That helps to reduce burn/char damage.
Below is a plan shown an example of what I mean (except the V-notch is not shown).
Also see the "Stiletto" series of Rocket Gliders on my website:
https://georgesrockets.com/GRP/CONTEST/ContestPlans.htm