Honestly I wouldn’t call Quezt, Nero Caster and Jeanne “weak”, they’re not the strongest but in just one month you got one of the best ST rider, one of the best AoE Caster and a unique Ruler who can help you during some hard challenge quest and/or boss fights.
Really, the only servant I would call “weak” is Stheno but just because there are better Assassin, she’s fine because every servant of this game has his/her own niche.
Even low star servants like Arash, Hans, Leonidas and Mozart can be very useful, you just have to understand their skills and what role they could play in your team.
So I suggest to lvl up and Ascend all your gold servants (start with the SSR of course) and don’t forget about some niche servants, like Euryale, who will be very useful during one of the singularity.
And, of course, if you have favorite servants, focus on them. Trust me, there are strong and weak servants in this game but ALL of them can be useful, so don’t be afraid if you are interested in a certain servant but he/she is a 1☆ servant