Performance will likely be really bad running on a live ISO since you are going to likely be bottlenecked by read/write speeds of the usb you have plugged in. I wouldn’t use it to test performance of games.

Yes but the games you download it will depend on your RAM (If liveusb copies it to ram) or USB

In a live environment I was not able too install too much - it always ran out of space, but I am not even sure what space it used, maybe a RAM disk?

So if Steam even fits with all of it’s dependencies, you may be able to try out a tiny game, definitely not 150GB Forza Horizon 5.

There are ways to make it work by using persistent storage, but it’s a hassle, at this point it would be easier to buy a 25$ 500GB ssd and install Linux on it.