I've found some other events like GDC Summer, New Game+ Expo, Indigo, Gamescom and Gamescom Asia, Gamelab
Viewing post in Dev Diary #16 - Beta Demo 24.06.20 comments
Although submitting is closed maybe try to contact Steam for the festival today and see if you can use the new demo to present the game. This event is one of the biggest this year and it'll be a huge lost opportunity. There is still time until it starts.
https://partner.steamgames.com/doc/marketing/steamgamefestival
You have the new upcoming demo, a steam page and etc, so you're pretty much eligible I hope to submit for the event. Miracles might happen, I hope!
Woah! Thanks so so much for the amazing suggestions & advice! Some of the event you mentioned are def on our list, and we have noted the rest too, this is super helpful! We have indeed gotten in touch with someone from Steam regarding a late submission just 2 weeks ago to which was politely responded to; that we were unfortunately unable to sneak in anymore at the time, perhaps we'll give it another shot!
Also, apologies for the late reply - we shifted gear crunching away, burning midnight oil to complete this demo update, there's a limited time to respond at the moment, but we'll try to check in as often as we can here! Massive thanks again for all the suggestions :D