Genuinely, it's the strange lack of specificity when it comes to smart casual for men that makes them seem so disparate.
Imo its closer to how you stretch that definition without going too formal or fully formal that makes smart casual for men work.
Also a little tip for people doing a tuck in, if you want it to look super clean, use garterized underwear and tuck your shirt in that before your pants. So instead of underwear -> shirt -> pants, you wanna go shirt -> underwear -> pants.