Ubuntu is bad. Go with any other distro. I generally recommend Fedora for newcomers. Specifically Fedora KDE Plasma Desktop edition: https://fedoraproject.org/en/kde/
If you fear you might break the system and don't have confidence in fixing it yourself, go with Fedora Kinoite. It's an immutable distro, so you can't break the system as easily as mutable ones: https://fedoraproject.org/en/atomic-desktops/kinoite/
While I don't necessarily like Flatpaks, you can start off by only using them for GUI applications. The most used repo is Flathub: https://flathub.org/
According to the dev and some people, the frontend is more performant than Lemmy/Kbin/Mbin due to the smaller data sizes. I read that it was because the dev came from some african (?) city where they only had like what, 2G internet connections?
As for the disadvantages, I can't say I find it great that it is written in Python. I don't like interpreted languages in production. That's all. Comparing the performance of it and the backend of Lemmy wouldn't be fair, since Lemmy's backend is written in Rust (compiled).
Yes. Though not all features are federated (yet), for example flairs.
Unless platforms start implementing the DID standard, this will be hacky to accomplish without weird artifacts. One other way is to do it how Mastodon does it, but that does not bring your content with it.
You don't have much choice in this case. Some instances will either not receive your account deletion request or just ignore it. Some instances might not even understand the deletion request. You can try contacting the admin of each instance, but I'm not sure whether you'd like to spend weeks contacting the admins of thousands of instances.
Ubuntu is bad. Go with any other distro. I generally recommend Fedora for newcomers. Specifically Fedora KDE Plasma Desktop edition: https://fedoraproject.org/en/kde/
If you fear you might break the system and don't have confidence in fixing it yourself, go with Fedora Kinoite. It's an immutable distro, so you can't break the system as easily as mutable ones: https://fedoraproject.org/en/atomic-desktops/kinoite/
While I don't necessarily like Flatpaks, you can start off by only using them for GUI applications. The most used repo is Flathub: https://flathub.org/