Sounds like you want a Raspberry Pi5. These can run a long time on a very small lithium battery. Cost about $80. I have several here in use inside 3D printers and robotics controllers.
But eventually, the need to have you own computer that you carry around will go away. Better to have toy data and desktop on the cloud, then you can log in from any random computer on Earth and have you data and desktop right there. The only reason we don't do this is because the Internet is too slow. One 2.5 gigabit speed it everywhere. There will be little need to carry you data around with you on a physical device.
This kind of system works today if you only need to move around within one building and you have the building wired for 1GB networking. I've worked in places like this, no one brothers to carry a laptop, they can use any random computer just as well
Then in another place most of the non-technical staff were given computers with ZERO internal storage. Nothing was ever stored on the local PC. It worked well enough that they never noticed. But you need very fast network.
But today in 2024, if you wanted to have mobile data, you could carry a 4TB flash drive with you and boot off that using any computer you have, You only need to carry the data, not the entire computer.
