What platforms exist for sharing/tracking sim trading performance or building a track record? I know of Collective 2.

I think there is fxbook ? never used it
Although not free, there is topsteptrader

While not a trading platform, Tradervue.com is an excellent journal that allows you to upload and share trades.

I just started with their service, the journal, reports and capabilities are fantastic and well worth the monthly fee, which is discounted for Elite members.

I'll be posting a review soon, it's definitely worth it.