How to make a "balanced" team

Lv up euryale, (Is usseless agains lancelot, but can lock charm gawain), and you can do the paid gacha (Im a F2P but this is the only gacha that i going to pay) to ensure a class other from berserker, the first wave include saber, lancer, archer and ruler (No serkers), you can wait to the Prisma Corps event for Chloe (Archer), the summer will be an assassin that can be usefull to battle Ozy (Rider), and hope get another servant.
From your serkers the most reliable its Heracles for his guts and evade, the other will die from a NP or normal attacks.

Speaking from very similar experience. (Its nice to know it's not just me out there)
Lean on your 2*/3* servants hard. Ushi, David, Hans, and the Cu lancers are my current go to. Ceasar and Shakespeare also have good damage support for berserkers.

With what you have I'd run Mash, Ibaraki, and support Waver. I'd have Nightingale and David, if you have him, and Herc as my anchor. I'd also have the Chaldea Combat Uniform as my mystic code. Mash and Waver should be able to keep ibaraki alive, add some damage buff, and fill the all important NP meter. David's party wide evade can help against a enemy NP and Nightingale can provide emergency heals.

Camelot is a no lancer go to, so that means Cu isn't going to be that useful there (especially that last boss fight). You can try bringing an OP support to carry you through Camelot like I did in JP, it's just that you need to play it correctly.

Otherwise, servants like David, Hans, Mash (super important), etc will be very useful for making your team solid.