Honestly, most things work and it comes down to personal preference. I've been sniping for a LONG time, so I've tried basically every type of secondary. Also, saying that AEGs have less range than pistols is just false and more often an AEG will have more range than a pistol, at least out of the box. With upgrades, they're about equal, but I think I'd still say a well-tuned AEG will out-range a well-tuned pistol. My order of preference is:
GBB pistol. Any TM pistol will do as long as you don't get rid of that lovely, super-efficient plastic slide. I've found they don't like it when temperatures are under 5 celcius, but much more and they still cycle no problem.
GBB SMG. My preference is an mp9 or mp7, something that'll fit in my SMG holster on my leg. Similar to the above, but can do somewhat reliable full-auto when it's warmer at the cost of being bulkier than a pistol.
Small AEG SMG. I have a Cyma mp5k and it's great. Shoots out to 60m fairly consistently with a new hop rubber and nub and gives me reliable full auto that will work in any weather. Only reason I prefer the other options is I like something I can holster. A small AEG needs to be slung, which means it has a tendency to swing around if you either move fast or need to crawl through the dirt or under bushes or through bushes.
Gas NBB pistol. The mk23 is amazing at what it does, but the trigger pull sucks. Gas NBB pistols are nice and silent, but awful for follow up shots, which is why I don't like it as much as a blowback; also the TM mk23 magazines are absolute wank, so you would need the ASG mags with better feed lips and possibly a better gas router so the cost goes up on that front. The silence is nice, but unless you're hitting first shot every time I prefer the ability to make quick follow ups and with how much a rampant twig or slight gust of wind has a tendency to ruin shots...
AEP. The dark horse option that has some great benefits (works in all weathers and often has full auto). Will easily hit to the ranges you need, but hit about as hard as you'd expect. Main issue is people won't always feel the hits from it, so you need to give them a bust of full auto, even at close range, which will make some people get all upset. I'd say about 70% of the time I've shot someone on semi with my TM mp7 AEP, they've either not felt it or not called the hit and I'm charitable enough to go with the former being more likely. They won't feel 1, but they'll feel a quick 5.
Full size AEG. If you're a big guy like me, it's possible to carry something like a regular M4 or AK alongside a bolt action without too many issues, but it's still bulky and entirely unnecessary in airsoft where something small will shoot just as well as something long, if not better because of how cylinder to barrel ratio works.
I don't like CO2 pistols because a lot of them tend to be more than 1.14J and I just don't like CO2 in general ever since I ran my VSR on it as I found it reliably inconsistent; the power will drop consistently as you empty the bulb, where green gas tends to be more consistent through the magazine, only dropping in power right towards the end, though it is more temperature dependent. Everything has its ups and downs, it just depends on what you want the most. I can't say what's best for you, as everyone plays this game differently and therefore everyone will have different views on what's best.