Also another name for Proton is Steamplay and the appeal of it, as the name suggests, is that it works out of the box after you tick a box in the steam client. There's the rub, it's a Steam thing.
There are ways to use Proton outside of Steam, but things get somewhat more complicated. A third-party client called Lutris makes it relatively easy, but still, it's not quite the same.
I'd guess it also works with regular WINE, after all Proton doesn't add that much, but again it's a bit more hassle.
In all of the above cases we as users have virtually no support and I don't know of any promises of support by devs or Valve in the case where we use Steamplay. That's the rub really. A lot of devs say the main burden of a Linux build is the support required and on the other hand neither evs nor Valve support a Linux build in case of Steamplay. The devs don't bother with a Linux build since it happens to mostly work through Proton, but no one supports it. The result is that we get fewer supported native builds. Yes, Valve fixes some issues for new AAA titles since those are very popular, but I'm not interested in those and Crying Suns isn't one of these games either. Sorry if I went of on a bit of a tangent there.
On my admittedly old Laptop (2012) that roughly meets minimal system requirements Steam Play is not able to run the demo. Probably because the GPU does not support Vulkan, thus not DXVK, which is required since the game uses DirectX 11. It's not my main machine, but it goes to show that I'd have no support if it were. Certainly not from Steam since it's not even on the official whitelist and probably not from you the devs either.
I personally strongly prefer to buy games with native Linux builds to the point where I rarely buy games without one. An exception are titles from the 90s or such because they have reached end of development and support anyway an Linux was a real niche back then. Those are the games WINE was made for IMHO. With new games on the other hand I buy those with native Linux support to incentivice Linux ports, hopefully.
Since it's a Unity game making a build shouldn't be too difficult (depending on third party assets you use).
There is also the relatively new and WiP https://gradyvuckovic.gitlab.io/linux-game-shipping-guide
/ with some useful information. As for support, I don't know why Linux users supposedly need so much support since we tend to be happy to help ourselves where possible and help devs if possible. That of course depends on how open you are with your problems, whether we have enough information to help you. So if you are inclined to try I am willing to help.