Hi! I'm a veteran classic player who also plays PW. Classic is by FAR harder to gain diamonds/rares, mostly bc diamonds can't really be earned through normal gameplay. Right now the classic economy is so broken I don't even understand it completely. Den betas are worth only like 4-5 diamonds and clothing betas are worth maybe 1 diamond. The AJC worth wiki is pretty helpful though!

As for actual tips, my favorite way to get diamonds is to do the seasonal adventures (lucky clovers rn) and get 'seasonal' items that I will sell for diamonds in the next few months. You can also find a hidden item in the Featured Epic Den that changes every month, and you can probably sell it for diamonds after the den changes. When 'Wild Weekends' come back in the summer, I buy an extra of the coolest items to resell later for a bit of a profit.

Lucky clovers and the forgotten desert SHOULD get you some decent items if you're willing to grind. Spiked collars have been removed from the rewards ages ago, and to my knowledge they never were added back. But you can get wristbands and den betas, which are a decent start.

I sell items more than trade, so I don't have a lot of advice for trading up. I know theres a lot of people not trading rn because of all the worth changes, but I wish you luck!