This is both a pro and con vs Windows, but definitely something to be aware of. There is a ton of information/documentation available. However, because of how flexible Linux is and the sheet number of distros, you may have to sift through a ton of information to find the solution relevant to your issue. This gets easier over time as you settle on the distro(s) and features you like. Filtering out irrelevant information will almost become a sixth sense.

A pi can definitely be used, but you may get much better performance for a similar price getting a refurbished pc, adding a second nic, and installing something like opnsense.

I’m going to be testing XigmaNAS. If I’m not crazy about it I’ll likely just run a pure install of FreeBSD