At least CalyxOS, DivestOS offer Android 13 builds for FP4 (and obviously LineageOS, but it doesn’t have OTA updates, afaik)

Well, IPx5 is technically water resistant for water jets and up to 12.5 liters per minute. I think that sounds enough to be used it rain. I also saw some reviews of other devices that even IPx4 is fine in rain.

In my setup I still use reverse proxy even though all of my services are inside a VPN. IMO it is just more convenient to have services accesible as subdomains or subdirectory than as different ports.

But pretty much in the same way as the YouTube’s frontend requesting content from YouTube’s backend. This is an equivalent of you loading a video on YouTube then going to developer tools and copying links from the Network tab. AFAIK all tools (Invidious, Piped, yt-dl) work this way.