I have built a number of the exact same model in both 38mm and 54mm. I've only ever used ProLine epoxy fillets to hold the fins on - no tip to tip - and have survived flights to Mach 2.4 (54mm) with no issues. One thing I've learned is to not put the fins so far forward as is typically shown with these rockets. Keeping them closer to the rear improves the stability by keeping the CP farther back (especially with the 54mm version).
I note that your Featherweight sled has two aluminum all-thread pieces that run alongside the antenna. I've done that with a single piece of the same type of all-thread and it definitely affects the radio signal. If you end up using that layout, you'll want to ground test your range and make sure you aren't losing a lot of signal strength and that you'll still be able to track the rocket based on its expected altitude. (As has been mentioned, you'll need to put the tracker in the nosecone, which is fiberglass, and not in the carbon fiber body tube.)
For motor retention, I've always used friction fit plus tape around the end, which works fine, but I realize that some RSOs won't allow friction fit on HPR motors. I've never seen that Apogee video before - that's a pretty clever way to do it. If you use the wire method, you might want to compare the amount of overhang of different motor cases to make sure the length of wire you choose will work with expected motors. (DMS vs. RMS) I'm looking forward to how you decide to handle this and the results.
Creating an anchor for the recovery harness is a bit of a challenge due to the small diameter. I glue a bulkhead above the longest motor I'll fly (which is either the CTI 6XL or the Loki K627 case) supported by a very short section of doubled coupler. (Bulkhead at the bottom, coupler on top.)
WARNING: The edges of the thin carbon fiber tubes are very sharp and can cut through kevlar as the rocket sways back and forth under chute. You must protect the kevlar wherever it comes into contact with a sharp edge. Heat shrink tubing works well, although I would use two layers.
For the 38mm kit, getting the chute packed is a challenge - don't be surprised if you really struggle with that and end up with a smaller chute, or something without a lot of seams. You also need a thin chute protector cut down to size. I think you'll have a real challenge getting a JLCR to work in such a small airframe.
With a larger J motor you should be able to hit 15,000' and get close to Mach 2. With the Loki K627, 20,000' and over Mach 2. I really enjoy the challenges of small MD rockets, and the Mongoose is my favorite kit. Good luck and enjoy the build.
Tony