Something I looked into a while ago was a means of moving a lot of people and equipment at a time. The answer I came up with is a space station right at the top edge of what can be jumped at one time. A Potemkin can carry six and a Monolith can carry two. Each station can carry 16,000 people with food and other life support rated for a year, plus over a quarter million tons of cargo. The price tag is approximately 1/30th that of a Leviathan and of course they can be built in much smaller yards. It would be relatively straightforward to alter the design to carry 40,000 people and only 30,000 tons of cargo. The advantage of these is that on arrival they can simply be left to be towed into orbit, without having to worry about delivering to a planetary surface or even planetary orbit.
Class/Model/Name: Transit Station
Tech: Inner Sphere / 3067
Vessel Type: Space Station
Rules: Level 1, Standard design
Rules Set: AeroTech2
Mass: 400,000 tons
Power Plant: Standard
Safe Thrust: 0
Maximum Thrust: 0
Armor Type: Standard
Armament: None
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Class/Model/Name: Transit Station
Mass: 400,000 tons
Equipment: Mass
Power Plant, Drive & Control: 4,800.00
Thrust: Safe Thrust: 0
Maximum Thrust: 0
Structural Integrity: 1 4,000.00
Total Heat Sinks: 142 Single .00
Fuel & Fuel Pumps: 1,548.00
Bridge, Controls, Radar, Computer & Attitude Thrusters: 400.00
Fire Control Computers: .00
Food & Water: (370 days supply) 30,000.00
Armor Type: Standard (606 total armor pts) 1,514.00
Capital Scale Armor Pts
Location: L / R
Fore: 101
Fore-Left/Right: 101/101
Aft-Left/Right: 101/101
Aft: 101
Cargo:
Bay 1: Cargo (1) with 10 doors 250,000.00
Bay 2: Small Craft (20) with 4 doors 4,000.00
Bay 3: Cargo (1) with 1 door 3,000.00
DropShip Capacity: 4 Docking Hardpoints 4,000.00
Grav Decks #1 - 16: (50-meter diameter) 800.00
Crew and Passengers:
21 Officers (20 minimum) 210.00
104 Crew (104 minimum) 728.00
1,000 1st Class Passengers 10,000.00
5,000 2nd Class Passengers 35,000.00
10,000 Steerage Passengers 50,000.00
100 Bay Personnel .00
Weapons and Equipment Loc SRV MRV LRV ERV Heat Mass
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TOTALS: Heat: 0 400,000.00
Tons Left: .00
Calculated Factors:
Total Cost: 937,397,340 C-Bills
Battle Value: 10,804
Assumptions
This version and the 40,000 person version are built in equal numbers. The Clans set up a command circuit of Monoliths/pairs of Star Lords from the Clan Homeworlds to a point in the near periphery that they can use as a staging area, reducing the distance to evacuate across. Allowing time for docking an undocking, this can reasonably reduce the transit time to 1 week, followed by a week of jumpships moving back in the other direction to reset. So two Transit Stations plus a dropship or three can be moved every fortnight.
That's still only 1,456,000 people moved in a year. Not very efficient.
So let's assume that all those jumpships instead are matched up with warships, carrying as many such stations as possible. Assuming fifty Monoliths, that's potentially 100 Transit Stations at once. Assume that the Clan Warship fleet can be converted for personnel transport, working on a basis of steerage accomodations and 400 days food etc that's 7 tons per person crammed into the cargo bays. (Ship numbers taken from sarna.net). Plus as many of the stations as possible on their dropship collars. Let's assume the 40,000 people 30,000 tons cargo versions.
24 Carracks (9,000) = 216,000
18 Vincent (13,000) = 234,000
3 Whirlwind (9,000) = 27,000
7 York (9,000) = 63,000
12 Essex (17,000) = 204,000
19 Lola III (15,000) = 285,000
16 Aegis (12,500) + 16 Transit Stations (40,000) = 840,000
6 Volga (27,500) + 6 Transit Stations (40,000) = 405,000
5 Congress (22,500) = 112,500
5 Black Lion (9,000) + 5 Transit Stations (40,000) = 245,000
14 Sovetskii Soyuz (29,500) + 14 Transit Stations (40,000) = 973,000
3 Liberator (10,000) + 3 Transit Stations (40,000) = 150,000
7 Cameron (25,000) = 175,000
7 Night Lord (23,500) + 7 Transit Stations (40,000) = 444,500
31 Potemkin (53,000) + 186 Transit Stations (40,000) = 9,083,000
5 Texas (41,000) + 5 Transit Stations (40,000) = 405,000
7 McKenna (36,500) + 7 Transit Stations (40,000) = 535,500
This adds up to 18,222,500 people and 10,470,000 tons of cargo being moved. It would require 349 transit stations, something that would need to be built up to over time, and could still only carry this many over a single 1 year journey, before then needing another year to go back and collect the next load (preferably with new transit stations built over that time).
It would take more than a hundred years (127) to relocate the entire Clan population (assuming that population remained static) and thirty years if Laborers are left behind. Still it's somewhat feasible.
And of course they need to keep a lot of manufacture - shipping mostly - in place to build all this shipping.
Probably the sane thing - since they can't just announce "we're leaving most people behind" is to declare that they're going to be relocating parts of their civilian castes to worlds closer to the Inner Sphere as staging grounds for the Invasion. They then commence the build up of shipping (converting warship cargo bays and building transports) while scouting for a suitable world or worlds that they can take quickly. Probably somewhere in the Periphery. They also begin a quiet cull of the civilian castes, secretly sterilising a proportion of the children assigned to the Laborer Caste (and to a smaller extent, members of other civilian castes). Advance expeditions with relatively balanced populations from all the Clans set out as soon as possible to start laying the groundwork.
With great fanfare the first major shipping expeditions set out carrying the frontline forces of every Clan, along with factories and shipyards to support them and (secretly) a large fraction of the warrior breeding programme and associated scientists. Two years later most of the fleet returns and reports that things are looking good, initial scouting indicates the Inner Sphere will have little chance of repelling them. It loads up another huge shipment of garrison forces, supplies and colonists for the 'paradise worlds' of the Inner Sphere (plus most of what remains of the warrior breeding programme). Quietly, more marginal Clan worlds are being left increasingly to their own devices - stripped of space transport and communication, the upper castes having departed, much of the population now aging and childless.
The invasion begins with the Clans using frontline forces and only those warships with minimal cargo capacity (destroyers and corvettes). The first year probably goes very well indeed for them - the Terran corridor of the FC is well fortified, as are nearby Kapteyn worlds, but this is before they've seriously upteched and they have no experience against the Clans so losses will be terrible for them. And of course, the two halves of the FC are divided, probably with the ruler of one side stuck on the other.
At least one more convoy will be sent back to collect more of the upper castes, leaving less-valued warriors, scientists and merchants to run the diminished Homeworlds while the rest of the Clan breeding programme, weapons research, and what manufacture they can't replace from their new acquisitions is removed. The Homeworld population will be beginning to drop naturally as the relatively high death rate exceeds the new, much reduced birthrate among the laborers. More worlds are dropping out of contact as they are stripped of spacefaring resources and abandoned. The convoys keep coming but get smaller and smaller as shipping is needed for relocating from the staging world(s) to the Hegemony and as shipyards in the Homeworlds are torn down and shipped off.
After twenty years and with 120-150 million Clanners shipped to the Inner Sphere, the last convoy leaves Strana Mechty, leaving no jumpships or HPGs and few if any dropships or shuttles behind. For that matter, fusion reactors and other valuable infrastructre have been taken so there's little in the way of power and water left for those abandoned.
The lights are going out...