Are you effectively synergising your party? Martial characters have multiple actions, while casters typically get one.
If you’re frustrated with 35% hit chances then you could focus on using some members of your party to debuff the enemy and buff the hard hitters; this has much better damage output than all 4 party members just slinging attacks with hit chances below 65%. If you want to just blast with all 4 characters then that’s a valid play but it isn’t guaranteed to be viable.
Probably not. It's my first time playing anything like this, so I picked druid. Bear for tanking, badger for knocking mobs around, and spider for AOE. Otherwise, using Wyll, Shadowheart, and Kalach. I'm not having much trouble, except for when the gith scout party glitches out and got involved in literally every combat no matter where on the map, but I rolled back a few hours and didn't let them spawn on the map yet. Just frustrated feeling so relatively weak, even if I'm generally winning encounters.
If you're winning encounters but it requires effort, I'd consider that pretty well balanced. The good guys don't always win if they make stupid moves. I decided I wanted to burn the goblin camp to the ground. I had to fight from rafters and use some cheesy tactics to win that fight. It was hard as fuck, but if you decide 4 people can take on an army it should be hard.
Weirdly, that was easy. I split up my team, targeted the wine barrels, iced up some of the camp floor, dropped Spike Growth, and just went to town. The hard part was that one guy inside on the throne. He had two attacks and action surge, and would one-shot whomever je targeted. The goblins were cake.
Managing spell slots is tricky, though. Have to decide carefully whether using any is warranted for a given fight.
Reevaluate your party and where you're attacking from(height, darkness, etc). I have a couple level 8 or 9 fighters that have multi-attack, so it evens out a bit soon enough. Under some circumstances, with a high initiative, multi-attack, then action surge, my fighters can debilitate an enemy before they even know they're in a fight.