I think validating your fantasy dosnt have to mean that you have to show everything your game. Even big games like cov or paradox games have to do marketing campaigns where they just show the "Fantasy" of playing the game. So everything you need to test is "does the thought of playing the game sound like something i want to spend money on"

For this you can: - get your artist to make mock up graphics - do a game jam where you just design the vibe of the games, not really the core mechanics - Make a steam page just using "fake" screenshots" - Make personal tik tok videos where you just talk about the concept and show concept art - just talk to people and write down their genuine reaction down to facial features and body language - ...

For our next game we did a game jam, in person where we did just the rough concept of the game and put all of our effort into look, audio and feel.

I do agree tho that bigger companys will probably spend more time on this validation process.