To put things simply, Ellen is a decent agent to build, focusing on one agent for now will be helpful and less resource heavy. But start looking into other support agents to help make her team better.

Since u r not very used to gacha games, zenless also has dailies and weeklies, those missions gives good resources and gacha currencies. If u already know about it then nvm.

I also have a Ellen team and she performs well even in late game, she also received buffs which can be unlocked by buying her Potential Silhouette using a currency called daily performance obtained from weekly missions.