Couple points.
1) Lava hardening nerf of magma defense: the lava is simply very hot and though it cools and solidifies into pumice, it could slowly harden in a reverse flood effect slowly going back to the magma river. Thus no worries about a legendary miner able to clean it out in a jiffy. Of course then you have to install new floodgates, dig new channels, and whatnot which takes a lot of time anyway.
The simplest thing to do is if you think the magma defense is too powerful - don’t use it. This is akin to people not bringing the outside river in because it represents bringing it uphill. You can do it if you want, but if you think it isn't ‘right’ then don’t do it. I think it unrealistic to flood the outside world with lava, so I do not do it. I have a trapped corridor with balistas at the end - anything that makes it through that gets finished off by whatever soldiers deigned to show up.
2) Given how unreliable soldier dwarfs are, invasions do need to be largely defeated by traps. If you could get your troops to actually answer the call to duty, stay at their station for a while, and thus be a reasonable force in a useful way, you could fight off an invasion with the military. Since they take ages to show up, do so in a trickle one or two at a time, wander off constantly to spend an entire season to fill a waterskin or sleep, and are generally unreliable for mounting a significant defense at your entrance it is necessary for traps to be a good defense against invasion. After the first couple invasions you will be hard pressed to survive without traps and moats.
If you want the challenge, then go ahead and play without traps or moats.
Since there are plans for a town outside the fortress, the more reasonable way to handle sieges is to have scouts warn you of approaching armies so you can send your own out to meet them in the field away from your fortress and town. If you do not send out an army, or it is defeated, the invaders then ransack the town before turning on your fortress.
This would be a serious drawback to just isolating yourself in a self-sufficient impregnable fortress.