Where are you looking? There are many more places to buy PDX games other than Steam.
How about their very own website? Here it is for 60% off. https://www.paradoxplaza.com/stellaris/STST01G-MASTER.html
Stellaris isn't an RTS, it's still very much a grand strategy. It's closer to Crusader Kings and Europa than Age of Empires or StarCraft, which is what people tend to think of when you throw the term RTS around.
The game is officially described as a grand strategy and I'd say it has sort of a mix of real-time and turn-based gameplay because it's not strictly either.
I know that Factorio is available direct from their site (and includes a free Steam key if you want to use it eventually).
If you can put up with Humble Bundle they have Starbound, with a Steam key as an included option.
Stellaris might be another pick if you're okay with PI's launcher (but I can't vouch for it).
Steam (has Linux client): https://store.steampowered.com/app/281990/Stellaris/
Paradox Launcher (has Linux client): https://www.paradoxplaza.com/stellaris/STST01G-MASTER.html
GOG (no Linux client): https://www.gog.com/game/stellaris
FYI: Stellaris is a "4X" strategy game with a Sci-Fi setting. You'll have different events and story lines every play through, and the writing shows their familiarity with tons of source material.
I don't spend much time on their forums, but this link came to my attention from a Tweet and I thought it should be shared here.
How about Crusader Kings II or the space based Stellaris. Another kind of aged game that is not quite strategy Mount & Blade: Warband.
I used the solar system map mostly as a background for Roll20 page and as an eye candy for my players. It was also easier for them to plan their actions when they saw all points of the interest in one place.
I used Stellaris to generate it. I know it's an overkill, but back then it was really easy for me to open a solar system, disable UI and save a 'print screen'.
...well, based on my Steam times, it's apparently CK2 > Stellaris > EU4 > HoI4 > Vic2. Though Vic2 is getting unfairly bashed for being a bit long in the tooth, I quite enjoyed fiddling with it.
CK2 has fun politics and makes for best stories and power fantasies about putting all the catholics in their place. Stellaris is the test range for cool features (though it does make it into a bit of a mess), allowed the writing teams to go wild, and includes the worm who loves you. EU4 is probably the most mechanically consistent experience, but at the cost of being less "flavourful." HoI4 right now feels like either spending time in micro, or cheesing. And Vic2 is a bit old, and, I mean, it's sequel has been out for a while.