Thanks for the detailed writeup! I've been troubled by the way that the server required for self-hosting is such a huge hurdle to get over at the beginning. Once you have one, doing more with it is easy, but I've often talked to friends who might be interested in hosting something who get stuck at that point. I hope things like this could help!
Of course, as you noted, many of these only really work on the same network. I still wonder if some clever new method of NAT traversal will come along that could help solve the problem.
I also wanted to mention, your comment about "...as you only need to keep one spare phone turned on and connected to your Wi-Fi." really got me thinking. Usually when someone is going to get started self hosting, they do it with a Raspberry Pi or maybe an old computer. But, could a phone do it?
A phone without a SIM card but on the Wi-Fi, old and so likely has a worn-out battery anyway so it's fine to always leave plugged in could maybe be a gateway to self-hosting for some people. I'm definitely going to think more about this!
As for your main question about using serverless systems, I've always found it to be annoyingly limiting when I try, but I would love for the tools to improve, because it feels so much more robust.
If you're in a position to ask the mods directly, I do think that's a good idea. It seems like a lot of places have had broad, sweeping policies trying to keep ordinary advertisers away, and norms about individuals like you or even just smaller operations haven't been worked out.
It's not quite the same, but I really liked this (very long, sorry) Mastodon thread specifically about how there's a lot of unexplored potential approaches to "commercial entities on the fediverse," and how we might be able to set some good rules and community norms. It might be too focused on Mastodon and on larger companies for your use, but perhaps it will be a helpful input?