Yeah, when I first saw events/challenges that I couldn't participate because I was lacking certain characters, I also felt tempted to get those characters otherwise I thought I might lose out because I couldn't get the rewards from those events. But I realized that I'll only be competing against people of the same shard as me, meaning people who started around the same timing as me. So if I do not have the characters, they most likely do not have it as well. Which brought my focus back to farming my arena team only.
One thing I saw a lot of people are talking about is the credit heist event, and I was very confused about it as well. Because as a newbie I couldn't understand the magnitude of the credits, 2-3 million seemed a lot and it doesn't seem like I would run out anytime soon, since it's mostly used to either upgrade my characters or to buy shipments. But after I joined my guild and saw them discussing about the cost of mods, that's when I realized a good mod can cost as much as 2-3 mil. Hence a lot of people are saying to focus on getting a scoundrel team to participate in the credit heist event. That's where the bottleneck would be in the future, with credits being the "limiting factor".
I agree with you, ST Han doesn't seem tanky at all when I started using him. But after that from what I read, the reason why ST Han is good is because it allows your other teammates to gain Turn Meter while he is tanking (taking damage). So it is possible for the rest of your team to have two turns to take out one/two of the enemy before they even have a chance to have their first turn, thereby weakening their squad by a lot.
Glad I could help! :)