1. The only problem with 121, the second 1 tends to whiff against some hitboxes, which is dumb as hell but puts you in a horrible spot.I've been playing a lot of Stunt Double recently, but I'm still pretty new to him overall. I have learned a couple things with him though that I didn't see touched on here.
1. i've begun to notice is people are catching onto 4/F3xxMimicxxMimic Kick as a starter after nut punch and armor through it, forcing to me eat a full combo. So to fix this (at least in the corner) is ending in 121. It has an insane knockdown recovery, so much so that you can raw Mimic and you're still plus. So you do your regular corner BnB but then end in 121, mimic, wait for wakeup, restart pressure with 2x mimic. I always end in nut punch if I want chip, but this seems like the best way for me to open up my opponent again and guarantee the corner pressure.
2. I've also learned that if you use mimics in your combo, you can end in 12(x6)xxNut Punch EX, 121 and be able to re-mimic because it takes long enough for your cooldown to reset
example: [1 mimc] (starter)xxDB1, NJP, 12, 12, 12, 12, 12, 12xxNut Punch EX, 121.
Its not optimized damage but I value having 2 mimics out A LOT in the corner.
2. If you're going to combo into it, use b2, b2 is a hard knockdown and is more plus on hit.