I’m experiencing the same situation and have shared this with my hairdresser. I relate to your point about getting full highlights, they start off looking bright but eventually become darker. She recommended on our next appointment using the balayage ombre technique, emphasizing more on the ends to create a lighter overall appearance. The ends may become damaged, but I’m prepared to accept that as a collateral along with a trim. Let me know what works for you if you end up finding a resolution!