You can of course export your yt subscriptions as a CSV file and import them into Piped, and you can also use a difference instance to host your account/data as the one you're using to stream the content. Did I mention built-in sponsorblock support and native apps like LibreTube?
It's not illegal, but maybe against YT terms of service. I've seen reports that google is starting to IP ban piped instances.
I'm taking it with a grain of salt anyway, as I haven't read any first hand accounts. And it would be interesting to see if google really is banning IPs, even ones in aws/azure/linode/gcp(lol)/etc IP blocks.