Paths of Glory: I have played Paths of Glory many times, along with its sister games; Illusions of Glory and Pursuit of Glory. The games are great, full of period flavor and history. However, they are based on a card hand mechanic, so probably not as good as solo games. Not knowing what is in your opponents hand adds a great deal of tension to the game. Personally, because of the card hand mechanic, I would not pick them for a solo session.
Empires and Alliances: I have not played Empires and Alliances but I have a copy and would be interested in playing a vassal game if you go with it and are interested. From the rules components point of view, it looks like a good balance between scope and playability. Also, I understand the game is based on AH's Guns of August. A game I have played and enjoyed...but that had a few issues. Anyway, if EandA is an update / upgrade of GofA, it is likely a good game.
Some other games you might also consider:
Fatal Alliances III: Compass Games by the Australia Design Group. I have a copy of this, but have not yet played. It is basically the WWI version of World in Flames. It seems to have plenty of chrome. The naval war, the air war, production etc. Probably worth a look at, but may be more involved than you are considering.
Balance of Powers: Compass Games, John Gorkowski design. The game has some great mechanics and allows you to play the naval war and the war in Africa. However, it is pretty complex and has somewhat high learning curve. Also, the color scheme and counters are done in kind of a "art deco" style and this irked some traditional wargamers. I have played it and enjoyed it however. Plenty of period flavor. Works well in solitaire mode.
Storm of Steel: Decision Games. I have no first hand knowledge of this game since it breaks one of my cardinal rules of game buying... it has no vassal module. However, I have looked at the rules and components, and it seems like a strong contender. The map is certainly one of the better ones with these types of games. It also has full implementation of the Russian - Turkish front for example. Only Der Weltkrieg and Pursuit of Glory really flesh out this front...so it is good to see.
Der Weltkrieg series: By SPW distributed by Decision Games. The "gold standard" IMHO. You can link all six games in a large Grand Campaign. It is a monster game, at 10km per hex, 1 week turns down to the division level. It has an interesting take on terrain that can turn off traditional wargamers and the map is cut in some strange places. While I would not recommend the "grand campaign", there are several "mini-games" in the series like the Schlieffen Plan that you might consider. Works well in solitaire mode. Also, I have only ever played on vassal, and those who have played it physically, complain of stacks that are sometimes hard to manage.
World War I by Decision Games, a Jim Dunnigan design: A S&T magazine game that graduated to a box. I have a copy, but have not played. It may fit your needs. It gets praise for simple easy design, eight pages of rules a single sheet of counters. Some grousing on BGG about play balance, but not an issue solo..
. There is also a vassal module.
https://www.boardgamegeek.com/boardgame/5297/world-war-i-deluxe-edition In the great games but crazy pricing category:
The Great War in Europe, OOP GMT. Ted Racier design. Would be a great choice. Simple, elegant design, an easy to play mini-monster, but priced way too high on ebay.
La Grande Guerre, OOP Azure Wish, Philippe Thibault design. Also a great choice, but priced way too high on ebay.
Honorable mention:
Glory's End and When Eagles Fight: A two-pack by GMT games. Ted Racier designs. When Eagles fight covers the main Eastern Front sections from 1914-1918. Nice easy to learn system that is fun. Still available from GMT.
