StarCraft II. It’s on Battle.Net though so you’ll have to download it through Lutris.

The first campaign is free and there’s a coop mode for casual players. Entirely free except for cosmetics, some coop commanders, and the other campaign episodes. It’s easy to pick up IMO but VERY HARD to master. Graphics still hold up better than a lot of games despite being 13 years old. Despite its age, I believe it’s the peak of the RTS genre. There’s are a lot of community-made mods/maps that you can play for free. You can even play a remake of the WarCraft III campaign there.

Other RTS games that I like:

• Age of Empires II (Steam). Graphics look decent enough for the remastered version on Steam. Complex economy system and slow rock-paper-scissors combat. Very slow-paced.
• StarCraft Broodwar (Battle.Net). Remastered graphics also look decent enough. Unit pathing might still have annoying quirks like the original. Slightly slower-paced than StarCraft II.
• Northgard (Steam). Beautiful game with a unique event-type economy system. Slower-paced than StarCraft II with some colony-building elements.

Not really an RTS game but I’ve been putting so many hours into RimWorld (Steam, native) this year. At its core, it’s a colony-building game but it does have some RTS elements to it (economy, combat, and unit management). Lots of mods available.