Honestly - using Sxmo on a PinePhone Pro is about as hard-core as you can go beyong building your own thing from scratch.
Ubuntu Touch on Pixel 3a, or Sailfish X on a supported device or a Librem 5 with PureOS even is a lot less geeky and more approachable thing.
Officially, yes, but there's also https://elagost.com/flatpak/ Also (shameless self plug): https://linuxphoneapps.org/apps/org.signal.signal/
I wonder how much additional work would be necessary to actually support A64, too.
But for now this is definitely good news to PineCube hackers/users ;-)
The easy route is to just wait for the merge request that adds the device to be merged.
You can also try using the pmaports branch that is going to be merged to build and install it - but I am not aware of a guide that explains how to do that and have no expertise with this, sadly.
The Pixel 3a? As stated on https://devices.ubuntu-touch.io/ it's the best port out there (and my previous experience with a OG Volla/GS 290 confirms this). Now is Ubuntu Touch perfect? No, and everything depends on your use-case (and because of no VoLTE support, the region you are in also matters). You can also try Droidian (https://droidian.org/) with a Pixel 3a, so given likely availability of a working, used device for less than USD/EUR 100 you can't go wrong with it, IMHO.
Writing this on my Librem 5 as a happy Librem 5 user, I struggle to find a good answer - maybe the Shift 6mq is an alternative (see the discussion on that in the equivalent to this thread in c/linux), as Shift have actively supported mainline development. The PinePhone is slower than the Librem 5, and the PinePhone Pro ... I could not daily drive it, too many bugs and too short active use battery life. If you just don't want to rely Purism shipping soon, you can always try a second hand Librem 5. Also: While I am quite happy, I am an enthusiast - YMMV.
If you like a headphone jack, and the 6 (no T) is also available in your region, I would go with it instead. Battery life should be fine, unless you leave battery hogging apps running all the time. My 6 definitely lasts from 7 a.m. (when I disconnect it) until the next morning when I forget to plug it back in - and that's with a Matrix account running in the background in Chats/chatty.
Yes, the Librem 5 is expensive and Purism treat consumers poorly.
But, the comparison and the focus on pure specs make it seem that you don't understand the appeal of the product, which is to run a GNU/Linux stack on a phone with a very-close-to-mainline kernel. Among the devices you compared the Librem 5 too, the only one that's comparable is the PinePhone Pro (yes, the others support Ubuntu Touch, but they are essentially standard Android hardware featuring a Mediatek or Qualcomm SoC. The vendor kernel is then being used with a compatibility layer to run Ubuntu Touch on it.
The PinePhone Pro (as the only other mainline smartphone in your comparison) is significantly cheaper, but that's in large part due to PINE64's modus operandi: They supply hardware, and the community makes that hardware usable by supplying the software. This model has worked okay for the OG PinePhone, may be due to the Community Editions, where PINE64 partnered with distributions/software projects, but it has not worked so well for the PinePhone Pro. The PinePhone Pro also has - depending on how you want to spin it - a too power hungry SoC or a undersized battery. Thanks to standby, it can last a day, but you can't really use it for much during that day then - e.g., browsing the web rapidly drains the battery. Also, without Purism's efforts, there would be way less user space software to make use of the device.
The Librem 5 is not without flaws, it's a really complicated hardware device (they were aiming for some FSF stamp of approval) - while the (socketed) 4G modem has GPS support, Purism also added a dedicated chip for that so that you can navigate while the LTE unit is "killswitched off". The NXP i.MX8M only has Cortex A53 cores, and the GPU is not amazing, either (at the time when design decisions were made, it was the only GPU with decent blobless mainline driver support though), but at least the battery is large enough to make the Librem 5 a phone I can reasonably use as a daily driver these days.
Regarding the Liberty Phone: I hate the name, but given that this is just the Librem 5 USA with as much RAM as the SoC supports and a larger eMMC, there's no technical excuse to delay that product, as these hardware changes are very, very minor.
I use Evolution for email (which ... well, is somewhat doable with scale-to-fit), and it notifies me about contact birthdays.
Mullvad supports Wireguard and works for me - I recommend setting up NetworkManger with a Wireguard config which is a bit involved, but afterwards it's well integrated into Phosh.
Very much not. GNOME Shell Mobile was funded by the German Prototype Fund in 2022 IIRC, way later than Phosh was created (funded by Purism for their Librem 5). GNOME Shell Mobile will eventually be part of GNOME proper (meaning it's Mutter, and GNOME Shell, patched to work on small devices), currently it's a patch set on top of multiple GNOME components that's packaged in postmarketOS and the AUR (if you consider AUR stuff packaged).
Phosh was created on based on wlroots (which is also used in Sway and other wayland-native window managers) and GTK3, as a Mobile Shell. Ironically, this way was pursued because Purism developers where told by the GNOME Shell people that an adaptation of GNOME Shell for Mobile would not be feasible.
Both rely on designs created by (at least then) Purism-employed designer Tobias Bernard IIRC, and thus may seem quite similar despite being based on a different tech stack, and both are hosted on GNOME's Gitlab, using all the same apps.