I agree. I am a new Dwarf Fortress player, I just retired my second fort after FPS death (~120 dwarfs on a 4x4 map, was getting down to ~45 and I didn't really have the heart to persevere), and I didn't really spoil myself very much. I looked up bridges in the wiki to see how they work, it mentions they're destroyer-proof so I used bridges instead of doors because that just seemed sensible, and then I was able to easily outlast all my sieges by just closing the bridge, having dwarves fire through fortifications, and survive on the absolutely copious amounts of food I could produce. It wasn't difficult at all; and certainly didn't match the expectations I formed from e.g. Boatmurdered. I'm just a little discouraged. I know that one can always go out and create active fun, but sometimes it can be nice to have "passive" fun as well, where you react to what the game throws at you rather than always initiating.
My suggestions are basically: let building destroyers smash thin (1-tile across, maybe?) walls and bridges; let non-dwarves dig at the very least dirt levels; encourage goblins to climb more; severely nerf traps; make goblins better warriors; and severely nerf farming.
Bridges are just OP, I don't think I need to detail why too much; but in general I feel like a sufficiently large invading force should have to be responded to with your own military. Anything which allows permanent turtling is a fun-destroyer, hence why bridge/wall-destroying, digging and climbing are musts. Traps are also just way too good, they decimate enemy front rows far too easily. Similarly, it takes ~3 years tops to have a legendary combat Dwarf, whereas goblin fighting skills don't seem to change, only numbers, so by year 5 one of my combat dwarfs could singlehandedly take out ~8 goblins with only a few scratches, it was disgusting. Goblin skills should rise with the wealth of your fortress, as logically goblins better at raiding will prioritize bigger rewards even if they're harder, whereas weak goblins would target easier rewards.
However, I feel like farming is one of the absolute biggest problems. I could feed a fortress of ~120 with ~5 planters without really putting much effort in on fertilizing. In contrast, in the European medieval ages, about 90% of the population was involved in agriculture, either at a primary level or processing the secondary products directly resulting from said agriculture. In Dwarf Fortress, it's about ~8%, maybe, and most are just unemployed haulers that feel useless. I want to be thinking "hooray!" when migrants turn up, not "meh, more useless mouths". Food is literally everywhere in DF. I'm not really sure what the solution is - needing more space for farms is definitely one, and I think needing to feed tame animals would be another. Maybe making fertilizer more complicated, and reducing plant growth time as well?
However, the broader problem is that specifically *underground* farming is OP, because ultimately it is what enables turtling. Even if bridges/walls were destroyable, I'm fairly sure I could whip up some sort of barrier given enough time and innovation, which allows me to turtle. In the European medieval era, you couldn't turtle forever (unless you were a coastal city) because you needed access to farmland - when your food stock ran out, the siege was lost. However, because I can have my farms *inside* my fortress, I can replenish food *during* a siege. This is a big problem, and I don't really see a way around it. Ideally, some sort of overground *only* activity should be necessary in some way to maintain your fort.