None of your listed use cases will even come close to taxing the 6600k. It’s going to probably sit happily in idle states most of the time.

Proxmox also has great snapshotting and backup features. Makes it easier to mess around with your containers/VMs without worrying too much.

Most people go with either FreshRSS or Miniflux but there’s lots of options to choose from.

https://selfh.st/apps/?tag=RSS

Any low power device is going to struggle with video playback unless you’re doing direct streaming. The exception being the new Intel N line of processors that have capable enough GPUs to do the transcoding. If all you’re after is decent transcoding performance for H264/H265 video, anything newer than Intel’s 7-8th gen CPUs will do the job. $100-$200 used optiplexes and thinkcentres can easily handle this type of workload.

For reference: https://en.wikipedia.org/wiki/Intel_Quick_Sync_Video#Hardware_decoding_and_encoding

Can you be more specific about your performance objectives? The Pi5 can handle jellyfin and nextcloud relatively well, so if you’re having issues with them, identifying the bottlenecks can help narrow down the hardware recommendations.