I believe this is the number displayed if you view the community on the original instance?
Debian is the classic server choice. If you don't have any server administration experience, I'd consider it just for that reason: there should be a ton of resources available. If you want something else, any RPM-based distro (like Fedora Server, CentOS Stream, Rocky Linux, or even RHEL) could be another option, with Rocky Linux probably being the best choice out of those.
Alternatively, I'd consider NixOS or Alpine. NixOS is what I use on most of my servers, however both have attributes that might make them worse for a beginner. NixOS uses a custom programming language to configure the operating system, while Alpine is much more minimal than most other server distributions. On the off chance that you have experience with a functional language like Haskell, though, NixOS might be the best choice, since it having a unified configuration for the whole system makes it very convenient for hosting usecases.
I'd also like to note that I run both a single-user Mastodon and Lemmy instance, and find them both fairly easy to manage. There's also GoToSocial, which is specifically designed to be easy to deploy.
The official web client works very well as a PWA on mobile.
The official native apps are also an option, but are controversial for a number of reasons. I don't use it, but from what I've heard Tusky is a very popular alternative.
Or does a self hosted lemmy instance federate all comments from all threads once it knows about a server?
Only in subscribed communities, but yes.
There’s no way to disable the compositor, so if you play any windowed games, you’ll have some extra input lag.
The reason compositors historically increase input lag so much is due to design flaws in Xorg. With VRR Wayland has comparable input lag to Xorg with no compositor, and it's only slightly worse than Xorg without VRR. In the best case scenario Wayland can have better input lag than uncomposited Xorg: there's a reason the Steam Deck uses Wayland in game mode.
I think as of recently Wayland with compositing might actually have better input lag than Xorg without compositing, but I haven't seen any thorough benchmarks in the past few months.
Wayland fixes a number of bugs and fundamental issues in X.Org. It is also much more actively developed, and it seems likely that over time new hardware will stop working with X.Org due to bugs nobody is interested in fixing (this is already the case for the Apple M1/M2 GPU drivers).
On the other hand, Wayland also introduces a number of additional bugs. However, some distros like Fedora have enabled it by default for years. In practice, most of the bugs relate to at least one of Nvidia's drivers, DPI scaling, and DEs with less mature support. If you're using an AMD GPU, 1080p display, and GNOME, you'll likely have few issues and possibly even a better experience compared to X.Org.
If it’s been opened and had anything replaced, unless they’re better than 99% of the DIY steam deck crowd, they left small traces of getting past the backplate.
For what it's worth, I believe the general consensus is that Valve cannot reject your warranty claim for the device having been opened under US law (the FTC has gone after other manufacturers for this).
On the other hand, I don't really trust Reddit with my upvotes/downvotes all that much more than random users, so I already refrain from voting on content I wouldn't want to be associated with....
Of course, Reddit can still see what posts I view, while that isn't the case for Lemmy (at least since I self-host an instance).