You don't even need to do the 12Z-plus deep pits at first.
Make a 1-tile wide pathway, with open space on either side, about 8-12 tiles long as the only way into the fort. This can simply be a section of wall that extends out perpendicular from your main wall with stairs on either end, or you could channel out (and then remove most of the ramps) a dry moat on either side of a pathway.
Populate the 1-tile wide pathway with 8-12 weapon traps using things like spiked wooden balls, training spears and junky wood/copper weapons. More traps with fewer weapons per trap is preferred over fewer traps with more weapons per trap. Maybe have a few cage traps at the inner edge to catch anything that makes it past 8-10 other traps.
Enemies will attempt to enter your fort via the 1-tile wide walkway and promptly dodge off the sides. Elves tend to get ground into hamburger by wooden corkscrews and trolls don't fare well either. And packs of rhesus monkeys... end up as monkey sausage.
If/when wagons ever get fixed... you would need to put a pair of 1-tile wide retracting bridges on either side of the 1-tile walkway so that the total width is 3-tiles when the bridges are extended. But right now, it doesn't matter (other then to make it easier on your dwarves). Or you could do a pair of 1-tile wide pathways and have the retracting bridge fill in the space between the two paths.
My entrance design is very much "keep the critters / enemies away from the door" at first. So I'll have the 1-tile wide dodge-this path first, then a few cage traps, then a sealable drawbridge, then a war dog on a leash, then a twist and turn, then a few more cage traps combined with pastured war dogs and another sealable drawbridge.
Later on I create a much more elaborate welcome area designed to make them dodge off the path, into a 1Z deep maze filled with serrated discs, corkscrews, giant axe blades, etc that gets more deadly as they get closer to the exit from the maze. And the stairs up out of the lower section puts them smack in the middle of a maze-like upper section designed to make them fall back down. Naturally, I have places where my marksdwarves can stand and observe / take pot-shots at all this. My goal there is to draw the siege/ambush in, but not kill them right away so that hopefully 2/3 or more of the attackers dodge off the path before any of them start dying.