Short answer IMO is that you want a relatively modern steel touring bike. A few thoughts:
- Don't get too hung up on mass is this is largely marketing-driven and other factors are more important - i.e. comfort and reliability
- Go for steel - it's typically more comfortable than ally and more resileant in the real world than carbon fibre
- Avoid CX bikes as these generally have "sharper" geometry that's quicker but less stable and will likely become tiring over large distances
- Reliability will be important, so look towards stuff running older / more basic and available standards - bar end shifters, 8 or 9sp cassette
In summary your priorities should be comfort and reliability; which is absolutely what touring bikes are about. A few bikes you might want to keep an eye out for - Dawes Galaxy, Genesis Tour de Fer, Fuji Touring, Ridgeback Panorama.. as well as offerings from Kona, Jamis, Surley...
Make sure you carry basic spares and support tools too - tubes, puncture repair kit, spokes, chain (or at least quick link), chain tool, multitool with allen keys and screwdrivers, gear and brake cable inners..
100 miles per day for 20 days is a huge ask - prioritise comfort, try to get in a lot of miles before the actual journey to ensure that the bike is well-fitted, broken in and reliable, and pepare yourself for the possibility that progress might be slower than you'd hoped. Good luck - sounds like an epic trip