Will Pop_OS run OK on an old Surface Pro

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

You certainly need the surface kernel, so check on github's linux-surface project for more.

It will run just fine. Almost every and any linux distro will work more than fine on a PC with these specs.

edit : I have a KDE distro running flawlessly on a 3rd generation i5 and 6Gb RAM. Sure, I don't use graphic-intensive programs on it but apart from that it's fine for work/everyday casual use

I would follow the instructions here: https://github.com/linux-surface/linux-surface, using the Ubuntu specific options for Pop_OS.

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).

It'll run pretty damn well. I've got pop on my min spec 2013 Macbook Pro, and, while it is slow at times, that's mainly because I've been too lazy to open it up and swap drives.

I got it working on an old Surface Pro 3 I had sitting around. Haven’t tested the pen, but the touchscreen and Windows button work fine for me, which is amazing! I just had to turn off secure boot or whatever to install.

if you want a little extra measure of performance, you can switch to another desktop environment (you’ll just be losing out on all that Cosmic goodness)

I recently installed Pop_OS 22.04 on a surface pro 4 (i5-6300U w/ 8G Ram). It runs well but I haven't tested the pen or put any effort into getting the touchscreen working. I just really needed it to not be windows because I was stuck using it for a week.

I'm running pop on a lenovo yoga x1 first gen. i7-6600u, 16 GB ram, runs great other than video decoding is a little resource intensive. As others have stated, bigger concern is whether other hardware (bluetooth/wifi/ etc) is supported.

Ps- stylus and touchscreen on my yoga work fine and show up as generic tablet device.

LinuxOSs, in general, are really power friendly. You will be fine.

Being Linux it's hard to go wrong. There are only a few distros that need a lot of power.