I use grapheneos on A P7 pro. Banking apps work fine because the bootloader gets locked with grapheneos. This is what most banking apps would complain about. Also if it for some reason requires google play services this is available too in a sandboxes way.
If you are afraid of being ddosed which is very unlikely. Cloudflare has free ddos protection. You can put some but not all things behind their proxy.
Also instead of making things publicly available look in to using a VPN. Wireguard with "wireguard easy" makes this very simple.
VLANs do not make you network magically more secure. But when setup correctly can increase security a load if something has already penetrated the network. But also just to streamline a network and allow or deny some parts of the network.
I prefer the opposite. I want the best quality I can get often 4K remux. Storage is cheap nowadays and I don't mind waiting a few days for a movie to download. Also I do have a 500/500 connection which helps.
the reason why arch gets recommend a lot as a gaming distro is that it is bleeding edge. Their for has very up to date drivers and parches that can help gaming. But with the current state of gaming on Linux this is a bit less of a requirement. most distros are new enough for most games. Exception might be debian LTS or something.
So i totaly agree that choosing something other then arch for gaming is a good option if you are rather new to linux.
Steam has mostly fixed the "service" issue compared to something like streaming services. One place to get almost any and all games. One account to access it all. Very simple for the end user. And does a load for the community as well. Take a look at proton for a example.