These are the current servants I have at the 4 and 5 star rarity. I’m really not sure on what I need to start to round out my roster. I’m a new player who only started with Anniversary event this year so any help would be appreciated.
From a glance, your roster’s biggest weak spots seem to be sabers, with lesser issues on the lancer and assassin fronts. I’d recommend Saber Lancelot or Wu Tian.
Alternatively, if you have some SQ to spare, since you have Merlin already, you could wait until it’s either Arthuria’s or Laltoria’s banner to pull there, as you might get spooked by them or, more likely, their alter versions.
I’d recommend Saber Lancelot or Caster Gilgamesh. They both cover a niche that you’re missing. Lance fills the saber niche, and Gil is a really good support that’ll help out plenty of your servants.
As much as I’d like to suggest Saberlot, Lancer Artotia would probably be the best pick for you. You lack any high rarity lancers so she would fit nicely into your chaldea, and the next event gives us a good ST saber in the form of Elizabeth Bathory(Brave) so Saberlot isn’t a necessity
If you enjoy Sherlock teams, CasGil is a natural partner for him.
Saberlot is another good, safe pick—and he has 2 buster cards, so good Merlin synergy too.
Nitocris is also always a safe pick but she gets a banner in a few days with Cleopatra—if you want her, wait until then.
Your roster is lacking in Wave clearers, so I’d also consider Zerklot or Fran.