If we're talking about recommended servants that can increase your gameplay value tremendously A.K.A using Cheat legally ( better becareful, Merlin is a gameshark/Cheat Engine of FGO ), you should probably save for Merlin. It seems that Merlin is the safest option here, as you can save more SQs for a pretty long time and increase your chance to get him. Like mentioned above, Merlin is the highest recommendation.
There is also Waver, he share his banner with Arthur Pendragon. Almost as same as Merlin, Waver is Waver and he is FGO OG Cheat Engine !
Gameplay wise, both are the strongest servants of FGO who always stand at the top.
There's no equal if we put a Merlin or Waver, they're simply the Great General of FGO itself.
Another option, cover your holes/each class department and roll for your favorite servants.
Bonus : you can calculate your saving in shipfu.moe - SQ Calculator. Good luck, farm hard and have fun 😁
The alter egos is a nice addition. However you also will get a free 4* single target alter ego if you don't want to waste your SQ.
From the look of your roster, a lancer is the best addition. There will be class based summoning campaign next month or you can try Enkidu in March. Summer rerun also have 2 limited lancer Tamamo and Kiyohime. Nerofest have Brynhilde pickup. Ereshkigal on Christmas
Caster is also another recommendation as you lack of the 3 top caster(merlin, waver, tamamo)