I am going to throw out a bunch of suggestions here with some comments about each. Note that it is kinda hard to really judge the learning curve since I have been computer gaming since about 1990, so 35 years of practice kinda skews my perspective.
4X games: since Civilization was mentioned, lets start with this type game, which are about starting small and "expanding, exploring, exploiting and exterminating". Usually turn based strategy.
Civilization 4: To my mind, this is the best 4X game, and the best game in the civ series. Probably fairly accessible and a ton of fun and cheap these days. Also could try Civ 7, the newest iteration, which I think would be more accessible than 5 or 6.
Stellaris: A space based 4X game, and real time instead of turn based. I include it on the list simply because there are some good resources out there to help you learn.
War Based Strategy Games: By these, I mean ones you move around the map for the whole war, not one where you have a map for each battle kinda thing.
Panzer Corps 2: Fight battles in WW2 playing the Axis side, with units that continue from battle to battle, but each battle is its own battle. Based on the classic Panzer General series, and very good. Probably the easiest learning curve of these type games, but still kinda steap.
Unity of Command 2: Essentially Panzer Corps 2 but playing as the Allies, and with more focus on supply. A little more complex, but still very good.
Ultimate General Civil War: A truly great game, and probably the lowest learning curve of any of these type games. The one problem is that the battles are fought in real time, but pausable to issue commands, so not too twitch type gameplay.
War Based Grand Strategy: Games where the war as a whole is the focus, not the individual battles as above.
Hearts of Iron 4: WARNING: This has a very high, steep, painful learning curve. You will need to put a real effort into searching out tutorials and primers for the game, but oh lord is it worth it. Play as any country in the world in 1936, and guide your country through WW2.
Fields Of Glory: I have not played this game, but I have heard good things about it. Grand Strategy in the Medieval era.
Squad Based Tactical Games: Games where you control a small group of soldiers through battles. When done right, one of my favorite genres.
XCom 2: One of the best games of all time, this game gets things right. Lead your soldiers battling aliens who have taken over Earth. You take 4 to 6 soldiers to each battle sight and fight off the aliens in turn based combat.
Battletech: Xcom, but your soldiers are giant fighting robots that are massively customizable. As good as XCom 2.
Other Strategy Games:
Factorio: Crash land on an alien planet, build, build, build, slowly automating everything, and building massively complex systems. Relaxing and satisfying.
Satisfactory: Factorio, but in 3d, with a simply stunning alien world.
Slay The Spire: A card game about battling enemies to get to the top of a tower. Cheap, and insanely fun. It is so easy to put 100s and 100s of hours in it, just playing one of two games a day. I still play a game almost every day after 4 years. Great value for your dollar.
I hope this helps some. I focused on strategy games as that seems to be your interest, but ask if there are other genres you are interested in, or if you want more information about a game.