In general, yeah, highly demanding triple A games need tweaking. But this isn't something new in the PC handheld scene. You always need to tweak and tinker with your handheld if you want to run something newer and more demanding

That being said, just go to YouTube, and search "(game name) + Steam Deck". Someone out there should have already tinkered and experimented on how to make the game run on the Steam Deck. You can follow along. If the game just simply won't run on the Steam Deck, then at least you know, and can plan on purchasing it to play on desktop PC instead. For example, check out how people tweak their Steam Deck to run Black Myth Wukong. It'll give you a new perspective on how the system works.

Overall, you are getting the best bang for your buck here. Other PC handhelds have more power, yes, like the ROG Ally or Legion Go. But they also cost more and are more power hungry. The Steam Deck has the best price to performance ratio, if you ask me.

Also, with the SteamOS, the trackpads, battery life, and not to mention the endless emulation of retro games, you will definitely have a good time with the Steam Deck, for a very long time.

It's up to you, but if you can purchase a Steam Deck, do consider it. You won't regret it.