FanchFilingCabinet
You mentioned you have an Asus router. Which one? Why not move to hosting your stuff on the router? https://www.snbforums.com/forums/asuswrt-merlin.42/ Sure it doesn't completely solve the issue but in my experience it's incredibly stable, and more so people expect to restart the router if the Internet isn't working which simplifies things too. Also beneficial is that you can give different clients different DNS servers comfortably.
Specifically, check out https://diversion.ch/ for dns blocking but its capable of a lot more.
Isn't it typically just kernel sources that would need to be released under GPL? A custom kernel is typically a few steps later in bringing up a custom rom for a device. The existing build is probably enough to try and get twrp running if you can get the bootloader unlocked.
Edit: All the same, very noble. Best of luck getting the sources, and making something useful of them.
Depends a lot what you're looking for in a seedbox. If you want to race then a lot of the options given are great. Personally I want storage capacity above most things and grabbed a dedicated server from Hetzner auctions. Today you'd be looking at 40TB gross storage for €60 a month. Been a long time since I've looked at setup options, swizzin was hot stuff then to give more of a "seedbox experience"