@philosophene just checking out this app on iOS - not sure it is what I want, but will post about it again if it helps. (Invite code attached to the URL btw)
https://tubebrowserpro.com/?invitedBy=ZNvxXsxKUYPpZph3fKP65fTcHda2

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@jan no problem, thanks anyway. I think I have found something for iOS - it's a browser app that has a YouTube ad blocker and supports Chromecast whilst hopefully avoiding notifications. It's not the clean UI or separate playlist management I was hoping for, but it's something and app looks pretty solid. I've shared the link here for anybody else reading. It has an invite code on the link - if 3 people follow I will get upgraded - just a full disclosure.
https://tubebrowserpro.com/?invitedBy=ZNvxXsxKUYPpZph3fKP65fTcHda2