ME1 is a bit simplistic, my go-to has always been to play 1 on Easy to focus on the worldbuilding and set up continuity (I suggest you spare people in 1 even if you play as a full Renegade).

After that you can move on to 2 and 3, on Hard at minimum, it's the best example of "casual hard" I can think of: since it's a squad-based cover shooter you have tons of tools to control the situation, but enemies will still gib you if you do deserve it. High difficulty is usually my kryptonite but anything below in ME really feels trivial and underwhelming.

Now I realise this is not high praise, but Mass Effect 2 and especially 3 are easily the best coop shooters ever. The guns are very varied, from typical bulky gray, to technicolor alien, from bullets to lasers to explosives, and have wildly different recoil and behaviors.

There's also a lot of little touches that make the thing more visceral, like the way weapons are stored folded and will deploy in your hand when you enter combat, or the empty casings hitting the ground. They are also loud and noisy, including the reloads: gameplay-wise it's a reload of course, but in-universe guns overheat, so "reloading" is actually swapping heatsinks, accompanied by a "pshhh steam discharge that makes it all the more satisfying:

https://www.youtube.com/watch?v=ApbVqQjTvv0#t=13m53s

Enemies react appropriately from getting shot, not the best in that department but certainly not disappointing either. Again, in easy mode enemies die too fast and you don't get to enjoy the gunplay, in harder modes you get to see them wince and stagger as you keep shooting until they die.

Since it's sci-fi it has a lot of freedom with enemy types: plain military, "magic" users, bots, monsters, etc. Spellcasting is also available to you and your teammates if you so choose, of course.

You can keep playing after wrapping up the main story, but it's not a great idea as you might have closed off story paths by reaching the end. Plus since it's a trilogy you would just be delaying your next game. Either don't fret the side-content, or be prepared for a long-winded space opera.