Sadly I don't think that's possible. Modern games are much larger and more complex than the traditional games that you could use save states in.

Not only would you need to save several gigs of RAM to disk, you also need to somehow capture the state of things like graphics cards and potentially the filesystem and other devices, none of which are trivial.

It could potentially work like save states in emulators do. Even if its only a single state like on XBOX, it would be so good if this works reliably someday.

A recent steamOS update said it was laying the foundation for hibernation, which should be a more battery friendly alternative to sleep.

The second part depends on the game, some will just work. Others need to run a first launch script and install some stuff before launch, which I agree should be part of the download process. Finally some games require license check or additional network access on first launch while the game is running, and this part can't be automated by valve. Overall it's just a good habit to always run newly installed games once, at least long enough to get to the main menu.

when I download games they still aren’t ready to play upon first startup, they need additional downloads/connectivity upon first launch

This specifically bite me in the past. Despite me knowing this being an issue, I completely forgot when preparing for a longer offline session. Some games need to run once to play them in offline mode, but I believe this is not required for all games. It's not an issue with the Steam Deck specifically, but with Steam in general.

Maybe instead running the game automatically, there should be an indicator ("green dot color" and not ready as "gray dot color") that the game didn't run and is not ready for offline play (at least from side of Steam). And even further, the indicator could stay at "No Offline Play" ("red dot color") showing the game cannot be played offline. This would be a setting by the publisher to show its not possible.

I'm playing 2nd chapter right now, and yeah it's such a great QoL feature. It's a real shame more games don't do something similar.

Just for the security part of it, you can isolate steam games from your file system. Some people prefer flatpak steam on regular linux because you can easily block it's access to system/user files and only let it access steam files and library directories.

For steamOS, you can create a fake home folder and add a bwrap command to the launch options, which will set system files as read only and only let it have direct access to the fake home folder.

Yeah, part of the early recommendation for using the pause games plugin was to increase your swap size to 16GB or more, to make room for running multiple games like this. Most everyone just sticks to the default ZRAM now, and even if you increase the emergency swap I don't think the system will use it until the ZRAM is maxed capacity.