Absolutely. The vast majority of players just play single player, it just has its occasional quirks. Only the most competitive sorts can't stand it, which is probably not you if you don't want MP anyways.

Do note this is a real time strategy game with 4X elements, so being good at Civ might not directly translate to this game at first if you have no RTS experience.

Random maps are always random, and will use any of the planets or other celestial bodies form the DLCs you get. Some of the "static" (fixed, or non-random) maps may use random planets as well, but a lot of them don't. The game has random planet lists that can be used even on fixed maps (the planet type is random, but a planet with always appear in the same location). So it really just depends, and there isn't an easy way to tell from in game. I mostly play random maps to be honest.

You can also make your own random maps with the in-game map designer, or fixed maps (which may used standard or random planet lists, your choice) with the galaxy forge tool located in the game install folder.

Gal Civ 3 is a 4X Turn base Strategy game like Civ, Sins is an RTS/4X hybrid. Sins is probably more unique and has better battles simply due to its real time nature (there is a pause button if you need to take some time to think/issue orders for single player). I haven't played GalCiv 3, so beyond that I can't say anything.

If you want the Sins AI to be a challenge, don't read the forums or ask for advice, because as soon as you understand the game and learn a few key tips, you will absolutely wreck the AI...

GC3 has significantly better SP experience and you can expect the AI to improve dramatically in the future, something that you won't see with Sins....Sins is a streamlined game with a clear cut counter system that facilitates a strong MP experience -- it lacks the depth to give the player the same SP experience that something like Civ or GC can give...

The AI can still be a challenge for speed-running purposes even once you understand its flaws. Offensively it basically can't beat well-supported starbases, defensively it can't handle a critical mass of bombers, and economically its expansion is anemically slow. Put it together and a game plan of "grab key strategic choke points, fortify them, build up a powerful economy, then mass bombers" is pretty easy to pull off against any difficult AI in any scenario. However, if you're going for a speed-run then putting up starbases is tricky without crippling your offense and a critical mass of bombers is just not happening in terms of affordability. While there still are some blatant abuses you can pull, bringing down an AI's homeworld in under an hour on a random 1v1 map is pretty challenging at unfair difficulty.

I can attest that Sins is pretty awesome in singleplayer despite the AI's flaws.

Nah, tried a but of EU 3 which I got from some humble bundle. Couldn't get into it. I have no doubt any Turn Based game will have more depth in things like diplomacy or research and other Empire building stuff, but each player values certain things more than others. So depth there just might not be as rewarding for a more RTS focused strategy gamer.

On the other hand, I couldn't get into the Wargame series, an RTS, because it was entirely about tactical, real time battle depth. It seemed if your units were in the best terrain cover or any of the other niddy gritty details, you would just get massacred. I don't quite like that Real Time Tactics end of the spectrum either. Sins is a fantastic middle ground.

None of the games I mentioned are turn based, they are all RTS...

It helps to have "plans" in mind that you intend to execute, as a lot of time can be wasted on deciding what to do instead of just doing it...

As an example, say you are playing as the TEC Loyalists on a roomy map (larger with empty player slots) with quickstart on...you know that fast colonizing is your first priority so you can immediately queue up an akkan capital ship (it can colonize planets), 2 additional scout ships, a colony frigate, and then a continuous stream of cobalt light frigates until you run out of fleet supply...you also can queue up 2 civilian labs so that you can research some of the technologies that unlock planets...that is all decision making that can be done before the game even begins, and will make managing all those things easier since you are not spending time actually making decisions but rather implementing them...

Obviously if you are new to the game and new to RTS in general then you don't yet know what decisions are good yet, but you can at least develop a routine and stick to it for several starts in a row...even if it is not the best strategy, the consistency will help you manage other things and take a lot of the stress out of it....refining your routines can occur later as you get better at the game, for now just focus on developing specific routines (colonization, tech beelines, etc.) and cookie-cutter fleet compositions (like heavy cruiser + flak frigate spam)...