Welcome to the game and the thread! That's a pretty decent starting team, and you'll find that a lot of servants are usable for all sorts of things. If you want to give me your friend code, I'll add you. I have a ton of servants leveled, so I could help you out with some strong support units.
There are a few servants at lower rarities that you will get plenty of copies of that are worth building:
-Euryale: 3 star archer that is super effective against male sabers, completely shuts them down with charm so they can't act. Really helpful for a few story and event fights.
-Cu (not Proto): 3 star lancer that is great for outlasting fights with that isn't a saber. Level him and his skills, put him last in your lineup and he can solo for quite a while without dying.
-Kiyohime: 3 star berserker that's good for farming until you get some 4* AoE berserkers. Berserkers deal extra damage to almost every class, so it's nice not having to worry about your lineup and just plow through a wave of enemies
-Arash: 2 star archer and king of farming. This isn't a priority until you get a a few max high level servants, but once you're at strength to reliably farm things, he's the #1 servant IMO. Get him as high leveled and as much ATK as you can, then level his 3rd skill. Then, preferably, equip a craft essence that gives him at least 70% NP at the start. He uses his NP, which is strong, fast, and almost always wipes out the first wave of enemies. He also dies afterwards which is great, since it allows you to switch in a different servant for the next wave(s). Your mileage may vary with lancers, but even with lancers if they have sub 12k HP should still be doable with the right stats. He is the #1 time saver on the more grindy events, and opens up a lot of opportunities to 3-turn an average farming stage (less wasted time). You will appreciate that later, lol.
-Hans: 2 star caster. I hear he's good for buffing your servants, but I rerolled to get Waver so he was never a priority for him, nor would I ever use him. That said, he does have some nicer buffs, but I don't know why you wouldn't just use a friend support Waver.
-Medea: 3 star caster. Has a very useful niche of removing an enemy's buffs that can be used fairly often. I personally haven't used her much, but I could see where it would be nice to have her around. Can also clear 1 ally's debuffs.
Also, you should try to unlock the
Chaldea Combat Uniform ASAP. It's a really strong master outfit (Mystic Code) that allows you to buff ATK, manually stun an enemy with really high success, and swap an ally for a backline ally.
Let me know if you have any questions about the game.