Here are my thoughts for running for a small group.
One, it’s a great opportunity for your players to really shine. So take a little extra time with each player every round in terms of your DM descriptions and the feedback loop.
Two, be prepared that the rounds will go by quickly, especially if you have less than four. It also means that if you spring too many enemies or super powerful enemies on players, the players don’t have enough manpower to heal up or deal with large waves of enemies at once. Their group action economy is limited. For that reason, and in light of number one, I wouldn’t throw too much at them too soon. Let them shine and feel like badasses as they chew through mooks and even fights for the first little bit until you get a feel.
Three, again, be prepared to TPK them easily if the dice get cold, especially if they don’t have a healer in the group. Have a plan in advance like having them wake up bound and gagged if they all drop. In fact, this is good advice for any early game.
Four, don’t throw save or die scenarios at them early. I don’t mean save or drop, I mean save or die. So, don’t have pits where a fall means certain death or monsters that can swallow characters whole with a failed save. That’s just setting your campaign up for an early end. Besides, those types of deaths are just no fun.
Five, like any game, build in opportunities where the players can roll checks and use their stats early. In my experience, when players are rolling dice, they’re gonna have fun.
Six, give them some early loot or leveling bonuses. That will build them up faster where they can take on more. Plus, the constant dopamine hits will keep them craving more from you.
Okay, there are six quick pieces of advice for small groups. I am sure the shield wall has additional tips as well.