Diplomacy is unusual in that it was designed to be played by mail. Importantly, the thing that makes diplomacy able to do this is not that the turns are simultaneous, but rather that it has a very low degree of what I will term "mechanical interactivity". Mechanical interactivity is a part of a game that requires input from multiple players for the game to be able to progress. This is distinct from strategic interactivity, which is that what players are doing is impacted by what other players are doing.
Diplomacy has one of the highest levels of strategic interactivity of board games I have played. However, the overwhelming majority of that occurs in abstract strategic and tactical reasoning and inter-player negotiation, none of which touch the actual mechanics of the game. To play a full year of diplomacy, players need submit at most 5 sets of orders (2 moves, 2 retreats, and 1 build/disband), and the game (either run by a moderator or digital platform) can handle all the rest since orders are resolved algorithmically and require no further player choices to determine the result.
Meanwhile, Game of Thrones is actually a very bad match for what you're looking for. While players do place orders in secret, orders are resolved one at a time with players making many choices throughout. It has a very high degree of mechanical interactivity.
Risk is potentially a very good game for this. As long as you have a fair platform for managing dice rolls (this likely requires a digital platform to run the entire game), Risk can be played quite well asynchronously, as there is nothing anyone can do during other players turns.
Carcassone could work, although there are a lot of turns to finish a game, so it may go fairly long.
I've also heard Through the Ages plays well asynchronously via an app, but I have not tried it myself.
Ultimately, the thing to look for is "good asynchronous games" rather than "games with simultaneous turns".
Note: I feel obliged to warn you about playing Diplomacy with co-workers. Diplomacy is an incredible game from a strategic and negotiating perspective. Done well, it can be really fun and potentially even forge life-long friendships. Done poorly, it can lead to long standing resentment and ruin friendships. It all depends on the personalities at play. The risk with co-workers is probably too high.