I've been using the BlackOps Pistols into BlackOps Smart Pistols so far for the quickdraw and for shield agents and have found them to be very effective against things at the Promotion 2-3 point. I'm currently facing MiB and Syndicate where they have lost their edge and not sure if its worth investing into Mass Driver Pistols or to hold out longer for Turbolasers and Gauss and continue using Blackops Auto-Sniper Rifles without a sidearm since I don't have shield agents anymore
There are multiple options after BO (smart)pistol era that depend on what you have obtained. For example, BO Macro SMG is a reasonably good choice and usually will punch through eventually, but when you get it depends a bit on RNG. Even if you don't have it, you can still mostly use sniper rifles. In particular .308 Cal has extremely accurate snapshot once the agents' Bravery has improved a bit (say 50,60+, while allowing for some movement.
I have never bothered with Mass Driver weapons for soldiers (the tech is useful yo research for other progress, however). Researching it requires capturing a live alien. But when you have done it, you can also progress down the laser path. If you play your cards right, you will have scavenged some laser weapons from MIBs. And you don't even need to research anything to use them. The lasers are efficient against armored human enemies, because most of them have some resistance to kinetic (requiring effective regular weaponry to be effective), but not lasers. So if you're experiencing difficulties delivering damage, those may be worth checking out.
As for MIBs, the best way to deal with them is TAC grenade launcher + gas grenades on open maps. Their crafts are also a goldmine. There is usually just one MIB outdoors, everyone else is cramped indoors, and they get out usually one by one. With a few dogs you may want to more or less rush to the craft and block the exit with reaction barking. Once they get out one by one, stun and/or melee them. Capturing live MIBs when you have not researched that specific type of MIB also gives you huge number of points. But watch out, you need to pick the location carefully. In the worst case one MIB will open a door while the other will blast you through the open door, if you pick a spot where they can do so.