The farm collapse was a large concern. Not my first collapse by any means, but it's round, 60 tiles in diameter with a 30x30 square center section and peninsula remaining at the surface holding the inner core. I've had problems in the past with large collapses being incomplete - some sections falling but not leaving their layers - as if they just disappeared. This went off without a hitch. Even though it's 3z tall, I intend to leave a full story of soil under each usable area. It seems odd to plant things with nothing beneath it. If I need the space, I may change my mind on that as it'll double the size of each of these areas. But there's 500x2 tiles of grazing area for the milker and shearer, and that can be doubled if need be. Should be plenty provided I stick to smaller animals - goats, alpacas, sheep - and avoid the cows and whatnot.
The next challenge is to make sure that the design is solid before tunneling out all of the rail/water/magma/maintenance lines. The fortress will be 2 levels with about 30 industries per level. The top level is food and crafts related to food - bone carver, weaver, etc. Pottery and sand may cover both levels since the sand/clay will be another, smaller collapse and need to be near the top, but their support industries will be down below - stone, wood, metal, gems. Both levels will have magma (when I get around to it) so they may stay up top and ship their wares down below. There should be enough space for that - there's generally lots of room between bunkers with a few exceptions like the non-production skills - military, bookkeeper, etc. that pretty much only need food and booze and such. Each level is about 150x150. So about a full 3x3 embark. It's a 5x5 map. The depot is outside the 3x3. The power plant will be outside the 3x3. There will need to be some big cisterns, also outside the 3x3. The river runs just outside the 60 diameter middle defensive ring, so tapping it will be easy. It freezes for a while, so that helps too.
There's going to be 4 main shipping lines. One on each level for food, booze, soap, clothing - basically the necessities. One on each level for trade depot goods - in and out. Two on each level for non-finished goods - food and craft items on the top, wood and stone items on the bottom. Anything that is at the end of a production chain heads either out to the other bunkers for their use (like roasts) or to the trade depot. Between individual workshops there will be dedicated lines to take load off of the main lines. Spinning/weaving/dying/clothing are all in a line, so the weaver can send goods directly to the dyer who can send the dyed cloth directly to the clothier. There's too many interdependencies to do this for everything, but there's hopefully enough of them to make the craft lines still able to carry the other bits. Getting stuff like thread and cloth to the doctors will be interesting - I'm not entirely sure it's going to work. There will be some exchange of lines from top level to bottom level - food, etc. Several industries are dedicated to the caverns - woodcutting, hunting, fishing, herbalist, trapping, etc. - and their stuff will generally have dedicated lines to the appropriate shops. They'll likely be the last to get set up because of the challenges of getting the caverns set up, and I don't know what level might have water, so fishing might have to wait until the lowest cavern is breached. I'm still hoping there's a magma pipe up high so I don't have to make an enormous pump stack.
There's going to be a thousand levers. Seriously, probably at least 8 per bunker, with 70 bunkers? Plus levers for the power plant, plus some powered rollers where distances are too far for dwarves to push, plus a whole mess of permanent switches. I think I have a plan that will auto segregate migrants, which I'll test on another map. My thinking is that there will be a bunch of rooms - maybe 40 or 50 to make sure there's a room per dwarf/kid/pet. There will be a designated meeting area that the fortress can't generally reach (behind a locked door) which I hope the migrants will path to once I cut off the main meeting area. Each room will be open to the outside and to the meeting area. Inside the door will be a pressure plate that will trigger a permanent switch which will lock the dwarf inside. The next dwarf will recalculate path, hit the next room, and similarly get locked in. This is repeated with all the migrants. A lever allows a separate door to be opened, freeing the migrant(s) from the room(s) so they can proceed to the bunker they've been assigned to via burrow, they cross another pressure plate which resets their room for use by the next migrant wave. I think it'll work provided that they don't pause to recalculate path for just enough time that two dwarves end up only a tile apart from each other and wind up in the same room. Animals path at different rates, but it's fine if they end up with a dwarf - they can be easily separated when they exit. Each room will be stocked with a bed, food, and booze (or a well) to allow time to get everyone sorted. Pretty much everything comes down to pathing working right here.