I’ve been running some external drives on my server for about a year now. In my experience, hard drives with an external power supply suffer less from random disconnects. The specific PC also makes quite a large difference in reliability. My server is just a regular desktop and has very little problem staying connected and powering my 3 external drives. My seedbox is an old laptop, and has been having almost constant problems with random disconnects and power issues. Maybe test how well your framework does with some external drives before committing to the plan?
Which compression level are you using? My old server is able to compress flac’s at the highest (and therefore “slowest”) compression level at >50x speed, so bumping the level up shouldn’t be too hard on your CPU.