Do you have an active leader? If your Alliance is treading water try looking here: Training Alliances or post a new thread asking for offers - you will get dozens of suggestions.
Use your backpack when possible to train uncommon heroes. The colour specific training is considered very poor value.
3* heroes are going to be useful to you for sometime so don’t discount them. But yes, park your 5* for a while.
8-7 is considered the best level for xp and recruits for 3 flags of world energy, but 5-4 is good for backpacks.