RTL8159 is great. It's almost the only USB 3.2 (not USB4, not Thunderbolt) to 10GbE network card. RJ45 instead of SFP+ is a bit frustrating, but at least we can reach 10GbE without requirement for more PCI-E slot or USB4/Thunderbolt support.
Raspberry Pi Zero 2 can run Linux. So you can run mail client on it of course.
What ever DisplayPort do, HDMI do more badly. HDMI also requires Annual Fee to get document now (since 2021, before that the document was public available). And this is not the worst thing, the worst thing is the HDMI forum disallow anyone open source a driver which support HDMI 2.1 or higher.
https://gitlab.freedesktop.org/drm/amd/-/issues/1417#note_2303163
It "converts" from and to Ethernet frames, of course. There is not actual L2 difference between PON network and normal fiber ethernet network. In simplified description, it just replace active switch with passive optical splitter.
Yes. Like fiber ethernet, differnet speed need different ONT. But noteworthy, there are some different standards for the same speed level, they are incompatible. For example, EPON and GPON, they are both for 1GbE but incompatible.
If you want to use microG as GMS replacement, it's impossible to do it without rooting. When application access GMS API, it will check the signature of GMS, so microG need some patch (Some ROM include it, like Lineage OS) or hook (e.g. via Xposed, like FakeGapps module) on system framework to return a fake signature for such a query.