Steam Deck

For the mouse, I recommend G305. It's wireless, but it lasts a truly stupid amount of time on a single AA. Just keep a spare in your bag and you will literally never have to think about charging it.

It has a fantastic sensor, and doesn't break the bank. As long as the shape fits you, it should be good.

For keyboards, look for "tenkeyless" or even smaller. Tenkeyless can come with full size keys, while being smaller by dropping the numpad. Even smaller keyboards might drop the columns of keys with the arrow keys and home/end/page keys, the function row, or even the number row. Somewhere along the spectrum you should get down to something that's about the size of a SteamDeck, or smaller, without making the keys you'll actually use while gaming, smaller.

If you want to save on thickness and weight, consider LP switches. Low profile mechanical keyboards have become more available. These'll be thinner and have shorter travel, but without going as flat as most laptops. They can be really nice, while also being way more portable than boards with full-height key switches.

I like them myself just for the ergonomics. A keyboard that lays flatter on my desk means less bending upwards and then back down in my hands and fingers when using it.

I use a G915 TKL, but that may still be a tad big next to the Deck. (And expensive)

Edit: I remember hearing good things about keychron. I don't have personal experience so do some research, but that K3 and this K7 seem potentially ideal. They also have a bunch of other models.