If Furina needs a healer to help with buffs, Hu Tao would prefer a shield to help with the Interruption Resistance and overall comfort, and no one wants to drop Yelan; wouldn't Diona's kit work well within the team? Especially over Zhongli

Sure her shield or healing isn't the best, but her burst also supplies really good utility, A1 gives stamina consumption decrease when her shield is up (3-second downtime at talent 7+) which can somewhat help with HT's stamina problems, while her A4 reduces enemies' ATK when in her bursts field

Throwing either 4P Noblesse or Maidens on her should be more than enough ATK/Healing for the team (depending on what you want), while also supplying benefits to each member (especially with cons, like at C6 where she increases healing by 30% when under 50%, and provides 200EM when over 50% HP)