I'm gonna recommend Northgard. It's a tile-based RTS with modern sprite graphics, as opposed to some games like Starcraft 2 or 0 AD (a free/libre game) that use 3d graphics. So, in some senses, it looks more indy or dated than a lot of modern offerings.

But it has really good gameplay, good base clans, good DLC, and is consistently updated. This can mean balancing/clan changes, but I must say that even when there was significant balancing action on my favorite clans, I come to accept and like the changes. Major DLC releases also come with content updates for the main game, and it really feels like Northgard is a labor of love.

It has a story mode that starts out easy-peasy and slowly ramps things up as you get better. It has multiplayer vs friends, random onliners, and/or bots of various difficulties. It has challenge modes per-clan that unlock new cosmetics.

Gameplay:

There's a lot of complexity here, but the campaign does a really good job of introducing it at a rate that's palatable.

This is a brief overview, mainly to show that, though Northgard may have simple graphics, it's really an in-depth and balanced RTS.

• Map tiles: you start with one, and acquire more.
• Seasons: Winter is a bitch. Always be extra-prepared for a bad winter.
• Random events: stuff to join in on or avoid. Rats, undead or ghost attacks, kraken, ceremonial bonfire, mine for the dwarves, exploding volcanoes that drop rock baddies that you can mine for stone
• Neutral factions: earn their favor to avoid their ire or to become their allies. Giants, kobolds, a wyvern, dwarves, myrkalfar - all have different potential benefits if you earn them.
• knowledges: learn and improve skills, gain clan specials, reduce costs - this is basically the tech tree.
• win conditions: domination, fame, economic, and even clan-specific or map-specific win conditions

Resources:

• happiness - earned through various means - nonphysical, not storeable - when positive, people work hard and new colonists come to the colony. When negative, people slack off and no new colonists arrive. Each colonist takes up happiness, except units that generate happiness, like bartenders.
• lore - earned by loremasters and various means - nonphysical, storeable - buy knowledges
• food - earned by food-producing units like farmers, fishers, hunters, healers, and idle colonists - storeable, infinite supply - all colonists need food to survive. Also used for colonizing new tiles and holding feasts (boost production and provide fame)
• wood - earned by woodcutters - storeable, infinite supply - build and repair structures. Keep people warm in winter.
• krowns - earned by sailors, trading posts, markets - storeable, infinite supply - money. Used in all kinds of areas. Used in training military units.
• stone - earned by miners - storeable, limited supply - building upgrades
• iron - earned by miners - storeable, limited supply - unit upgrades, hero spawn
• military experience: level up your military along an attack, defense, or strategic path
• fame: level up clan skills
• economic influence: it's complicated

Regardless of all of that complexity, Northgard manages to sum it all up into engaging, understandable gameplay, with clear stats on who's made the most progress on win conditions, etc, and a straightforward and manageable learning curve.

You can undergo cascades when things don't go right - a winter freezing without wood leaves everyone in poor health, often unhappy, performing at a fraction of their usual production - thus nobody's needs are met and everybody stays unhappy and underperforming. A 'simple' fix is to make everyone into regular villagers again, then farmers, woodcutters, healers, and other roles as you can afford them.

Also - Linux native.