TopRamen:
just so you know your lengths are very reasonable but the line weight doesn't have to be so heavy to do the job and outlast the model it's installed in
Yes it is very possible to use Kevlar only for your shock cords as long as they are indeed Long enough to allow the Nosecone to slow down before hitting the end of the cord.
Normally for most of my LPR & MPR models with motor mount anchored shockcords I'll start with 36-48" of Kevlar but Not anything nearly as heavy Strength wise as suggested above, usually 70-90lb Twisted or braided for models Up to about 2.5lbs and 100-130lb braided for models 2.5 to 3.3lbs Lift off weight. To this Kevlar is overhand knotted another 36" to 42" of 1/8" Oval elastic for LPR and MPRs up to about 2.5lb or, 36" to 60" of 1/4" flat elastic on larger MPR and Clustered Upscales. So you see the overall shock-line length of 10' to 12' isn't that extreme.
If using ONLY Kevlar than 10 to 12feet of 70-90lb or 12 -15feet of 100-130 for MPR models has proven to be just about right. This give the ejected nose cone plenty of Slowing down travel time without letting the body catch up or jurk it back creating "the Dreaded ESTES Dent".
Heavier MPR or HPR models would require longer lengths.
Years ago I purchased a roll of 250lb Braided kevlar that I've never found a good Rocket use for. I think I made a horizontal teather sling with it for my UpScale BT-80 size Orbital transport but I could just as easily used the 100 or 130lb braided Kevlar.
Heavier Kevlar lines May be needed on Larger Heavier HPR models but it is just to heavy and stiff for anything I fly.
Just about all my current fleet of Micro models use Kevlar only shocklines of 50-70lb x 30" to 36". Haven't lost a Nosecone or zippered a body on any size model over the last decade or so.
Why one may ask do I still use Kevlar/Elastic combinations? Actually I'm using Stainless Leader/Kevlar/Elastic combo's on all my LPR and MPR models now but that's another story.
The reason for the combination is to allow some stretch in the line to help lessen the opening load on the recovery system. by Placing the attachment point Butterfly knot about 1/3 of the shockcord length below the Nosecone it helps greatly reduce the opening shock on the chute while allow the Body and Nose to decelerate without bounce back.