Yes, you are missing the fact that Rust doesn't have const generics. What it does have is called const generics MVP and that MVP moniker is not just there for show.
What you want to do is perfectly achievable on nightly, but as you can guess there are no guarantees that what's working today on nightly would continue to work tomorrow.
It'll probably work fine for your use case, but keep in mind that I've had to ignore the incomplete_features lint for a reason - more complicated LENGTHs might trigger an ICE.
On the plus side, you'll almost certainly be able to use the same code when the feature is stabilised because that generic_const_exprs feature flag just enables extra behaviour in the compiler without touching syntax.