My network has reset several times and I’ve narrowed it down to an apparent DDOS attack
It's not. You will need to lower your torrent client's incoming connections limit and/or set lower limits to your incoming/outgoing bandwidth in your torrent client. It is clear your network router is unable to handle too much torrent traffic hitting it at the same time, hence the issues you are experiencing.
For qB
Tools / Options / Connection / Global Maximum Number Of Connections
Tools / Options / Speed / Global Rate Limits
There's no specific number to enter there, you just have to experiment a bit and set lower numbers until the problem goes away and your network is stable.
shutting down the client doesn’t help
It will, eventually. It does take a bit for other torrent clients to realize you're no longer online and stop sending you traffic / sharing your IP with other peers.
Was MakeMKV ever claimed to be open source?
Not sure if it's exactly what you want but I've used MKVtoolnix in the past for .mkv operations, worked fine for me. And ffmpeg also works great for general audio/video stuff though I've never tried bluray -> .mkv with it.