I’ve been running Pop!OS on my Surface Pro 6 (i7-8650U) for a while now (with the linux-surface kernel!), and touch/pen support is relatively good; only weird thing is the limitation around what the pen can do; e.g. you can touch-scroll with your fingers (and other gestures) but you can’t use the pen to scroll; the pen behaves more like a mouse, e.g. it will highlight text instead of moving the page (which makes sense)

You shouldn’t have any issues with installing the base OS; the keyboard cover should just work; but pen/touch doesn’t work out of the box, so to get that you will need the linux-surface kernel. I use the Debian package repository (as Pop!OS is a Debian or Ubuntu derivative - they use the same packaging system). Note that you will also likely want to tell the package manager to “hold” the installation of the non-surface kernel, using apt-mark hold. You’ll probably want to hold linux-headers-generic and linux-generic. (I think I have particular versions held as well.)

I quite like to use the “Xournal++” app for touch/pen friendly writing/drawing, and have also used it for signing PDFs (though only the visual type of signature, not the digital type like Adobe does).

That depends entirely on how well the firmware is supported on Linux; and which chip it uses for WiFi.