First check with your state laws an see what your options are. A sheathed knife can be very impracticable depending on the way it’s carried and the manner in which it’s sheathed. Here in Texas it’s pretty much no limit so length and mechanical don’t come into play. I’ve carried assisted opening but now carry a Boker model 74 auto. Not expensive and very reliable. I also carry either a 239 or 226 Sig, both in 9mm. See what your state allows and go from there.
I think my state (Maryland) takes the cake for incomprehensible knife laws.
For instance, one cannot carry an automatic knife with “intent to harm others”
How do you prove intent before the fact?