You recommend not to tank with him - I wonder how you're gonna avoid that on a melee character with no reach?
Furthermore his pathetic reach (particularly with Reduce Person) means he's usually only attacking once per round.
Delay his turn, then attack a flanked enemy. Give him what AC you can but don't try to draw agro with him. I don't consider inevitably drawing aggro sometimes to be tanking. Even my backliners Linzi and Jubilost draw agro at times. The thing is they aren't trying to. They aren't tanking. There's a difference between losing initiative and getting rushed and running into the thick of things to take hits for the party. The latter is tanking, the former happens to all characters. He'll get hit a couple times when this happens but will survive and kill that enemy, then go back to doing what he does best, stealing tankier characters kills, critting multiple times a round and triggering outflank. That's not what I'm gonna do, it's what I've been doing with Nok Nok.
I'd consider a character a tank if they're supposed to draw aggro. So Harrim's not a tank even though he gets attacked semi-frequently. Valerie is an AC tank. Amiri is an HP tank once she levels up a bit (and a DR tank as well with a respec to rager). Regongar is a mirror image and displacement tank. Animal companions and summons can be tanks. If I tried to draw aggro with him he'd die pretty quick, but so would most of them. It's not their role. I don't bitch that he can't heal either. He's a damage dealer. That's why you give him what AC you can. +2 ring and amulet, +4 DEX belt and +5 bracers are always available to me when I get him, then he very soon gets +4 DEX and CON. I also cast blur on every melee character with Jubilost. Until he gets the CON belt bear's endurance covers this. It's not much of a wait. With shield spell (which everyone gets buffed with in my party, except shield users I also cast blur on every melee character with Jubilost and blur (on every melee character from Jubilost) everyone gets by. Tank AC is only needed for tanks.
Draw agro with Valerie. Then send Nok-Nok in. Rogues aren't tanks. I'm sure someone could make a rogue into a tank but that's not what their class naturally does. Some enemies are immune to sneak attacks yes, but with all his attacks and keen weapons he kills them anyway. Giving him combat reflexes and outflank, and later the opportunist rogue talent makes him even more of a dirty kill stealing bastard than he already is. Enemies just explode whenever someone hits them, if Nok-Nok is nearby. Also I play with CoTW and cast the long arm buff on him with Jubilost. That isn't needed but it's fun to do.
I also use Amiri in the "goes second to deal damage" slot in my party, switching out with Jaethal when I feel like it. But Nok Nok fills this role very, very well.
My party almost always consists of front line tanks to take hits (I prefer Val and Amiri because I like them as characters)
Second line damage dealers. I like enlarged Harrim for this as well as my main is normally in this role (enlarged or long armed).
Reach helps here but isn't actually needed. It's the benefit of playing harder fights in turn based. You can choose who goes first more easily, and where you put them. You get a sense for where the AI will go on their turn so you stand the damage dealer on the left instead of the right (for example), because the right would draw aggro. Nok Nok gets better at this as his build progresses, but he can do it immediately. Rogues are meant to be played sneaky, not stand in front. I'm sure cleverer players than me stand their rogue in front to great effect, but that's the beauty of this system. It allows for novel, ideosyncratic solutions. But it's not the obvious way to use Nok-Nok. If you wanted reach on him in the unmodded game you could still enlarge him. He'd take a small hit to his DEX but he has so much you'd barely notice. Not needed though, with sensible play. I don't like reduce person.
Back line support (Linzi and Jubilost). Other characters can fill these roles.
Not to mention some enemies are immune to sneaks.
Immunities? Seriously? "some enemies" are immune to necromancy, some enemies are immune to mind effecting. Some enemies are COMPLETELY IMMUNE TO MAGIC. Except alchemists every class has their cryptonite. A sneak attack on an enemy that is immune to precision damage is still an attack. Nok Nok will struggle against an enemy immune to precision damage AND critical hits, this is true. Otherwise he'll trigger outflank and kill them.
Amiri is a much stronger character in practice.
Either you're using Nok Nok wrong or I'm using Amiri wrong. Don't get me wrong, she's fine. But 16 STR isn't amazing for a barbarian. She's solid but not exactly a powerhouse. That coupled with being unable to wear a STR belt unless you want to put up with fatigue from raging limits her damage potential a bit. She's good but not great. I do use her sword unlike most which nerfs her somewhat. It's roleplaying.