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"

In terms of bang for the buck, I'd absolutely agree. It's only when a company fully depends on the income of a single client, or closely aligned few, that this becomes a question.

It's worth remembering a lot of these megacorps do employ people directly to work on FOSS projects. Here's a quick and lazy example involving AWS
https://redis.com/blog/redis-core-team-update/ but Red Hat and others do the same.

I'm not a fan, and it feels almost as if by employing and embedding people in these projects they look to exert control over them. Realistically, I don't see that as any different than if they were paying money directly for the same control. Except this way FOSS still has benefits after the license change.

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.