There's a lot of rambling here but your points are centred around battery life so I'll hit that.

The Steam Deck's battery life isn't amazing.
The most I've seen is Stardew Valley running for 5 hours.

There's two sides to this; the Steam Deck's power consumption settings and how long you actually want to spend playing.

The Deck allows you to control how much power a game is allowed to consume. I'm playing Fallen Order atm and that game would probably run for 2-3 hours of continuous play. I'm playing on mostly medium settings with textures on high.

There are certain other games that will utterly drink battery life, like Baldur's Gate 3. I would personally argue that you shouldn't be attempting it as a regular mode of playbut I tried it myself and it doesn't look terrible. But you have to go in understanding that BG3 is a graphically intensive game and the Steam Deck isn't exactly built for it.

On the second point, I personally can't spend more than 2 hours of continuous play. After a while, I get tired of holding the device. I live in the UK so our travel times aren't long. I also don't travel very often either. My time on the Deck is usually just before going to sleep. The way the device is, I'm not sure that you could be playing one game for that long. Sure, you can play Titanfall 2 but that's a very fast-paced game and it's probably not going to translate well to the Deck. It's better if you plug in KBM but then you're having to carry those around as well. At that point, you may as well be on a laptop or PC.

There are certain games I've decided that I'll play on the Deck like Fallen Order or No Man's Sky. I've also got emulators loaded if I ever want to play those games. If you want a Steam Deck, you should be getting it because it allows you to play a huge breadth of games, not the latest ones at high graphical settings. If you want that, get a PC, you'll be happier with that than the sacrifices you'll have to make on the Deck just to get the game to run well.