Get a good mouse with some extra buttons and it makes the whole experience a lot nicer. And remember you can usually change any keybindings you dont like. My mouse has two extea thumb buttons and theres some games where ill use one for sprint or dash, although left shift feels pretty natural after you get used to it for a while. The biggest difference with mouse is turning like you said, its not like a conteoller where you can just hold it to the side and then release to ‘reset’. Regardless of your preferred sensitivity tou have to get used to ‘resetting’ the mouse yourself by pucking it up and recentering it. But once you get used to that it feels way more responsive and accurate than controller aiming imo. I got a steamdeck a couple years ago and its largely replaced my pc for gaming but theres still times where i wish i had a mouse, even with the touchpads.
The genie is out of the bottle now. I love SteamOS with its ease of use and steaminput integration, but theres already a ton of open source projects building on what they started that would totally still be able to out perform microsoft even if valve stopped updating steamos today. But every indication seems to be that thats the opposite of their plan, as they’re extending steamOS support to other handhelds and devices officially now.