That sounds fine. I would only say don't use Syncthing to actually make your backups. My preference and recommendation is restic, possibly combined with the helper utility autorestic.
NPM likes to eat the let encrypt requests which is what I'm assuming is breaking the cert gen inside the container. I believe you can work around this, but honestly I'd recommend just moving to a more advanced but more flexibile proxy solution.
Personally I recommend Traefik. There isn't a friendly gui to help you but once you wrap your head around it things just work. It also allows for defining proxy parameters right in your compose file via labels so it takes out the need to log into NPM and manage proxy entries there. Just deploy you're compose fils and you're off.
As far as making what you've got just work, you can either try to get NPM to stop intercepting the LE cert requests or hack up the signal-tls-relay container and jam the NPM certs into it. I wouldn't recommend either of these options though. I've been in a similar scenario and it's this among other reasons why I moved off NPM. I started with NPM because I thought it would be simple and easy and it is, right up until you want to do a thing even slightly outside of its fairly limited box.
And as someone who uses Linux for literally everything I know better than to preach as if Linux is a full replacement for Windows. It's not and there are absolutely reasons to still use windows. OP isn't asking about the OS and given the nature of the question probably isn't ready for such a switch in the first place.
To my knowledge there is no such thing available however you have just enlightened me about TS6's featureset. It sounds like it is the exact solution you are asking for (and one I'm going to immediately try out myself.)
I don't pay any mind to example compose files. My are all quite custom anyway. Only thing that matters is paying attention to changelogs and watching for breaking changes.
You're requirements are too vague as "lots of apps/VMs" doesn't describe the expected load. Overall though if you want small just build a mini-ITX system. Then you can put in any x86 chip that fits your needs.
Can't say I've run into a need for such consideration yet. Excluding stacks explicitly meant to work together to some degree most of my services are an island to themselves and I like it that way. Then as far as notifications are concerned pretty much every supports at least email or ntfy.sh.
Well there can be some "risk" depending on how you're going about this. I'm assuming you will be wanting people outside of your home network to be able to each your server. To do so you'll either have to open a port in your LAN firewall and expose your server on said port to the internet, or have all users who will be using this on a VPN you create.
The former being "more risky" but quantifying that risk is difficult. Ive done this in the past and don't personally see it as a big deal. My current mumble server does not live on my LAN but I will be pulling my server out of a local data center in the nearish future and running it out of my home once more at which point a number of publicly accessible services will be hosted from my LAN.
The short answer to "can you add it to your home server" is yes. It's not like there is some cap beyond your own system resources that prevents you from running multiple services.
Yeah I didnt abandon Plex because I couldn't afford it. I have a number of gripes with plex but as long as it remained free I had no strong motivated to get rid of it. Now that I would have to pay though I have no interest in keeping it around. I am quite happy with jellyfin even if it may lack polish on some of its facets and I regularly accept inconvenience to uphold my own operating philosphies.
That sounds fine. I would only say don't use Syncthing to actually make your backups. My preference and recommendation is restic, possibly combined with the helper utility autorestic.