So if a Diamond Valka Favored is hurting your win rate, I think you should sell it, especially if you can turn a profit now.

You are being too penalized for playing at a low rank, I think you'd earn more with a better deck at a higher rank.

Playing in Solar Gold or Diamond with a worse deck full of Diamonds can be more profitable than playing in Mythic with a meta deck, but in Auric and below the multiplier takes a huge hit, so I think the first thing is to try and stay in Solar with the best deck you can afford and shine it up as you go along.

Another common advice from many people is to shine up the good Core cards first. Gold Core cards are quite cheap and you can work from there and slowly shine them up to Diamond.

Most of the Core cards you have in your deck are really strong and you'll probably use them for a long time, so it won't be a wasted effort.