(Note: The picture below is not really helpful)

This is the command I'm using, summon minecraft:falling_block ~ ~ ~ {NoGravity:1} (I'm using a repeating command block set to always active and unconditional)

When using this command, the block barely gets shown, it might flicker a little, but the majority of time it doesn't show at all. Here is a link to a project that uses these commands ---> https://www.planetminecraft.com/project/fnaf-security-breach-rtx-showcase-trailer/. If you test this in any 1.16 version compared to the current 1.17 version, you will see a big difference. Hopefully this post helps you and others as well.