the question is: why do you want to move on from the Rpi4? Also, how do you plan to handle video decoding 0f x265/av1 streams? Do those miniPCs have a video card with hardware transcoding capabilities? If you plan to do it on software (i.e. CPU transcoding) the CPUs will run hot most of the time.
you don’t need to know how the ISP network works, you only need some networking concepts. Subnets/addressing and very basic routing (for a basic setup). You won’t even need firewall rules if you don’t host anything at your place (that needs to be accessed from outside)