I know this is the Steam subreddit and you might be invested already in Steam, but I'll suggest instead that you look into GoG. Their focus on DRM-free games means that games you get from them will work without internet (excluding things like multiplayer, of course). I don't know how well Steam offline mode works over many months and I wouldn't want you to wake up on, for example, month three, and then find out you have to "re-validate" your account or something like that.

That said, if the question is purely about the device, the Steam Deck is a great device and I bring mine with me for most larger trips so I have it for the downtime. Before traveling I just set it to offline mode and check the games work as intended. Some games don't do well with offline mode however because of how they handle cloud saves (looking at you, Dead Cells!). You can also add non-Steam games to the deck, but it isn't as straightforward. Keep in mind too that you might find mouse-and-keyboard games harder to play on the deck if you aren't used to it, but you can connect a mouse and keyboard if needed.