Who should I invest in ?

For 4* berserker, Frankenstein for last wave farming (due to stun self 2 turns after NP)
Ibaraki for boss killer
Mashu as tank or leonidas since u already ascend him (but for long term Mashu is better)
For 4*Caster, Helena is better than Nursery in team composition
For 4* saber, Rama is good enough but if u didn't have enough "ascension material",use Caesar. He's the best 3*saber.
Kintoki is one of the best 4* riders and also no need to talk about Jack the ripper.

Rama and Jack for sure. Rama is a buster crit servant and Jack is the premier crit star generator. Pair them with Leonidas to provide buster buff and tank or Shakespeare for an even stronger buster buff and np gain. Or Hans for even stronger crits, more crit stars, and a nice heal/atk/def/crit gen buff. Once Merlin comes out he'll be the best fit for this team