I use the 10-15GB rule. If the install size is under 10-15GB, it's perfectly fine on an HDD. Optimal? No, but definitely workable and you'll find most installs - especially indies - fall into that range.

10GB is about 10,240 MB. Realistically, only a fraction of that will load at any time anyways. With a 160MB read speed (7200 RPM drive), the max read speed for 10,240 MB is 64 seconds. Even if that adds a minute to the initial startup, it's probably all loaded to RAM anyways. If it's a 5GB load (let's say the game is divided into 2 equal zones), is about 32 seconds... and that often is pre-initialized or major assets like player models are one-time loads on startup.

There's really no reason to put any 1GB or less installs on an NVMe. Unless 6.4 seconds bothers you.

Given that 16GB of RAM is fairly standard, it's highly unlikely for 16GB to fetch at any given time. Things that load have to go somewhere and that's typically RAM or VRAM.

I never really thought about the math and threw an old HDD with some files I wanted access to into my build when it had only a 500GB SSD. I honestly thought it'd be fairly useless for gaming, but then I remembered that I used to install CS:GO to my HDD because there's little need to spawn into warmup rounds quicker.

Some 10GB games will be flawless. Some with mini-map teleportation will be a huge inconvenience. I remember Palia seemed a bit inconvenient especially 'cause it made friends wait. But it was mostly tolerable.