With both major web-based YouTube frontends broken, there's few options for clean YouTube watching on iOS. Here are some of the alternatives I've found while Invidious and Piped are broken. This list contains only trusted and (mostly) open source solutions, no weird sketchy apps.
1. Duck Player
The [DuckDuckGo app] (https://apps.apple.com/us/app/duckduckgo-browser-search-ai/id663592361?platform=iphone&mt=8) lets you open YouTube videos in a special embedded player that blocks ads and removes all distractions. You won't be able to see the description, comments, and recommendations, but you might consider that a feature. Duck Player doesn't let you download videos, but it works with all YouTube video qualities up to 4K and even HDR. It also allows background playback and picture-in-picture.
2. Brave Playlist
In the [Brave Browser app] (https://apps.apple.com/us/app/brave-browser-search-engine/id1052879175), you can use Brave Playlist. You can also play videos directly from the YouTube website with up to 4K resolution, PiP, and background playback. But you can also add videos to Playlist, which will download them and let you watch them offline. The interface for this is clean and minimal, with no distractions, just like Duck Player. However, the video quality in this mode is atrocious. For video downloads, I recommend option 4.
3. Official YouTube app/website
If your only goal is to avoid YouTube ads, you can simply change your location using a [VPN] (https://thenewoil.org/en/guides/less-important/vpns/) to one of the few countries that do not have YouTube ads. These include: Albania, Cambodia, Ivory Coast, Laos, Myanmar, Macau, Madagascar, Maldives, and Russia. You can use the official app to do this, but know that it will be much less private than any of the other options, as it will allow Google to easily track your YouTube activity on your device, even without an account. You also still won't be able to use PiP, play in the background, or download videos without paying for Premium.
You can also just use the official website with AdGuard in Safari, this should block most ads if you update the filters regularly, or you can combine it with the VPN trick. Again, no PiP, no background play, no video downloads.
4. Cobalt
Cobalt isn't a video player, it's a download tool. You also need to download a video player such as VLC or Infuse, or suffer with the built-in video players in Files or Photos. This option is also very minimal, nothing but the video. You can choose the video quality and format in Cobalt's settings (I suggest sticking to H264 on iOS devices, though). It also works completely offline. Depending on your video player, you get background playback and PiP. However, it is very cumbersome and requires you to download the entire video before you can watch it. Another drawback is that there is no option to download subtitles. For offline viewing, this is the best option other than using yt-dlp and copying your files over from a computer.
In conclusion, there don't seem to be any complete alternatives to YouTube frontends. All of them have serious drawbacks or rely on the official YouTube mobile site. Unless you're completely against Google because of their privacy issues, paying for YouTube Premium is probably your best option. Or better yet, avoid YouTube on your mobile device and stick to using it on your computer. There are plenty of other ad-free streaming services and content available on your phone.