The simplest, perhaps, is "a frontline", as described by Sethatos, and is best suited for a small (10-20 troops), well-trained, well-equipped military.
I agree with most of what you wrote, but would like to say that this tactic above works well for any number of squads. I use it frequently. As Sethatos mentioned, given well-equipped and trained squads, I find exhaustion or separation becomes your primary killers, and
maximizing the number of bodies on the front line alleviates those. The more bodies you have, the less they will get exhausted (fewer drawn-out 1x1 combats), and the less dangerous it is once they do become exhausted. Since the dwarves seem to happily fight even when packed multiple to a tile (and with no apparent combat detriment that I can notice), I regularly throw 40-50 dwarves into spaces like these and watch them gobble up large invasion groups like Pacman. It becomes even more important if you add in enhanced invaders from mods.
Formation-wise, the game just doesn't have anything (yet) besides what amounts to a mob of 5 year olds chasing a ball around a field. Given that, tactically you want to create the tightest grouping of as many dwarves as possible, and consistently put it up against the most divided up grouping of invaders as possible. The fun to me is in the fort designs such as the ones Sver mentions, and coming up with clever ways of achieving the ball vs string that I mentioned.