To provide "personal recommendation", an App would most likely need to profile you, i.e. track your viewing habits and trends, and forecast your areas of interests.

IMO this goes directly against the concept of privacy.

On the brighter side, I think Freetube (local, on windows) and LibreTube (on Android, but needs a piped account) can provide personalized video suggestions, but you yourself should check them out.

A privacy focused client that offers personalized recommendations is a bit of a juxtaposition IMO, and is definitely impossible now that YouTube removed homepage personalization for logged out accounts.

If you are fine with YouTube without ads, then you could log in with the likes of Revanced or Grayjay, and get a feed with personalized recommendations. This isn't private though. Grayjay does however have a feature to anonymize your watch activity, and to perform certain actions in an anonymous YouTube session.

I'd even maybe be fine with something that takes all the watch next recommendations of the videos in the subscriptions page and jumbles them together as "recommendations"

NewPipe (and Freetube the last time I checked) already does this, but they use a built-in subscription system that does not require any kind of account

Piped (and LibreTube) uses its own account system which allows you to sync its built-in subscriptions & playlists between mobile and desktop, but I have not looked at whether they offer a feed with the synced subscriptions.

Personally if I'm specifically after watch recommendations, I just look at the related video suggestions on something that I like

Invidious and Newpipe do have watch next recommendations that aren't only subscribed channels, so there's a bit of discovery there, but no recommended type page where it aggregates those listings. As others have said, that would require tracking algorithms.

I have not found one that isn't having performance issues. On Fdroid there is LibreTube. I think it might be worth while just opening a separate YouTube account to just watch there (mostly if you care about commenting). I wonder if instead of having a front end it might be worth while to just have a plug in that keeps bookmarks on the channels you are subbed and a number on new videos added that month.

Sites

Youtube piped instances (what libretube uses) and Invidious instances

Android apps

Libretube (best option there is when it actually decides to work), grayjay (Louis Rossman's cool project), newpipeXsponsorblock (ol reliable, never seen it not work)

Idk how Freetube works but the recommendations are usually relevant to me and I don't have a google acc