If you climbed to Diamond 5, you are Diamond 5. You can do it on your main account, it will simply take longer because no doubt your MMR is fucked so you're gaining less LP per win. You gain massive amounts of LP on new accounts in the beginning so your climb is much faster.
Thing is, you have improved. You just didn't realize it. I don't personally feel like I'm doing anything different today than when I was hardstuck in B5 but when I reflect on it, I can see little things I do differently. Right now, I'm hardstuck Silver, and if I were to get placed in say, Gold on a new account, I'd certainly fall right back to Silver eventually, ruining many people's games along the way. Because at my current skill level I cannot maintain a Gold ranking. As much as I like to quarrel about feeding teamates, trolls etc, I know in my heart that the ladder system for the most part works as intended. I am a Silver player because, that is the highest I can climb. You are a Diamond player because that the highest you can climb.
If someone is hardstuck Bronze 4, they belong there. I started B5 last two seasons and managed to climb back to Silver both times with relative ease because that's where I belong. I have no doubt you can climb to Diamond on your main account in future seasons because you've proven it's where you belong. The MMR reset at the beginning of the next season would make it as easy as it was on your new account.