You wrote: "If I end up with Scarlett, PS5, an AVR and my PC connected to the TV, what happens if I want to also connect a future Chromecast/Roku/<insert streaming box here> down the road? Needing 5 HDMI inputs is going to limit your choices."
I this case you need 4 HDMI 2.1 inputs on the TV. Your connections will look like this:
TV: AVR, PS5, Scarlett and PC
AVR: Chromecast/Roku/other video streamer