A decent, recent Nvidia GPU is going to beat most CPUs. I wouldn't shell out extra for a GPU just for transcoding, though. Good enough is good enough.

Nextcloud is great. I don't doubt that OP is having problems, and I understand how frustration can set in and one might throw in the towel and look for alternatives, but OP's experience is atypical. I've been running it for years without any issues. I should point out that I only use it for small-scale personal stuff, but it's good for me. I have it syncing on eight devices, including Linux, MacOS, and Windows desktops; Android phone; iPad; Raspberry Pi. My phone auto-uploads new camera photos. I'm using WebDAV/Fuse mounts on some machines. Everything is solid.