Depending on the model you have, it might be possible to move some stuff to a microSD card. Ultimately if you're that close on storage you'll benefit from freeing up space because it has to constantly search for scraps of space to store stuff even temporarily which is a major performance drag.
also if you have a newer model it might be possible to replace the SSD with a larger one.
My Surface Go 3 is low on hard disk space, but that's because I have two operating systems (Windows and Android) installed onto it. I keep the Documents folder and the like on my SD card, along with some apps. Symbolic links is a godsend for moving files from the hard disk to the SD card. Fair warning, there's a learning curve!
It's an x86 and not an ARM Surface (if my terminology's correct), meaning that it uses traditional computer architecture. Meaning that unlike most tablets (but like most desktops and laptops), you're not stuck with any one operating system. You can install Linux, Android (an x86 distribution), BSD, OS/2, whatever you'd like. If the OS will install onto a typical desktop computer, then it should be able to install onto an x86 Surface.
Of course, a non-Windows OS on a Surface won't always be smoove sailing! It's best to use a distribution specifically meant for the Surface (such as the "FydeOS for You" Android) to minimize problems.