On the other hand, it it's too hard/complex, proper fortresses will be stifled.
But that's the point - Mining
needs to be stifled. Mining is too easy. It needs to be less easy to dig down to the magma sea while it's still the first month of the game.
In the original 2d version of DF, the game had barriers like the cave river, the chasm, and the magma river, which all were stumbling blocks that also presented ramping up of the challenge of the game. You needed to be prepared for the threats they posed (in the form of more monsters that would be difficult to block, and would require military dwarves to handle) and to construct bridges across those roadblocks to advance.
Now, the only thing even remotely like that are aquifers.
We have three caverns in the game right now, but there's no real reason why, since you can easily bypass caverns, so there's little real sense of danger in them, or sense that getting there is any form of achievement.
Likewise, easy mining makes a joke of sieges, since I can easily move mountains to build zig-zagging bridges to cross into my fortress in open view of marksdwarves who have all the time in the world to shoot at goblins as they fall off into dodge traps.
If, however, players are forced into making little more than holes in the rock and dirt before they start getting their first sieges, and maybe making wooden pallisades for defense because above ground construction is (rightfully) faster than excavation, then sieges as a whole become more challenging.
The game needs to have mining
much slower to make all the other challenges of the game actually challenging.