For Linux devices, if you have a powerful enough phone (Pine64, can we please have a really beefy PinePhone?), you could bundle Waydroid + Aurora/F-Droid.
From the original list, I really don't care at all about 1/2/3 (phone calls) and only care vaguely about SMS.
My requirements are for an excellent browser and an app store with support for messaging platforms (telegram/signal/skype/slack/teams/etc) and email support, everything else is either "extra" or an outright distraction I'd probably be better off without (twitter, reddit, etc).
I recognize that's true for you, but it's definitely not true for me, and I expect many others, too. I'd honestly be fine if my "phone" dropped the "phone" part entirely. The only purpose of that feature is for spammers to make my phone vibrate a couple times per week. In the very rare (less than once per month) instances I need to actually talk to a human on the device, I'm sure someone could invent an app that allows for that.
Any newcomer, Linux or not, would immediately face the most obvious problem, which is a complete lack of a well-stocked store.
[0] I sometimes take my smartphone abroad for a combination of offline Google Maps and WiFi hopping.