No, in 3 levels of the expedition, I have to go through some (fewer than 6) of the encounters by negotiation and am able to win the remainder by fighting. Perhaps I advanced too fast.
Depends on what you wish to achieve as to whether it was "too fast"
The higher up the ages you go the harder it becomes. If all you wanted to do is get through the Continent Map then you wouldn't need much, but if your goal is to gain more troops than what you lose, then getting a attack boost high enough to reduce losses will be essential.
Pairing troops with Rogues is also going to do wonders for troop retention for the age you're in.
I would 100% recommend you get a Arctic Orangery if you haven't already.
The enemy's troopers usually take the first attack and target my Rogues (if there were any);
I'm thinking this is the biggest reason for your losses.
Your posts are telling me you're not getting many Rogues, and are (quite likely) trying to produce too many types of Units.
When it comes to single unit production buildings (from the build menu), try restricting your selection to a single unit. The only other single-unit production building in your city should be Rogues
The reason for this is so your Alcatraz will produce solely those Units and nothing else.
Then in battle use that 1 unit type + rogues. It will stretch so much further than if you're constantly changing your unit typings.
For Modern Era you should be using 1-3 Tanks + Rogues (experiment a bit to see what ratio works at what Attrition level as by Modern Era the Rogues may struggle to keep up with the other Units on the Battlefield), and use the Unattached Units you gain from other sources (ie: Random Unit buildings, Guild Expedition Rewards, Guild Battleground Rewards etc) for when you know the Tanks aren't capable of defeating the opponent or would match poorly
I use my 1 Main Unit typing to do the main fighting, while I use the other Units as cannon fodder to throw at the enemy when I'm at the edge of how high I can get in GBG Attrition, PvP Arena and anywhere I don't care about retaining Units or know there'll be high amount of unit losses.
This is why Alcatraz is necessary, because no player in the game, no matter how high the attack and defence bonuses are, would be able to sustain without getting units from the Alcatraz.
Not true.
I created a city to do a never-build-a-Alcatraz challenge and found it incredibly easy to sustain without a Alcatraz. That city now has 10,000-ish Unattached troops + 6 Attached Rogues. It does take deliberate conscious decisions to make it work at the start of the city life cycle, but it can be done. You do obviously need a Unit source, it just doesn't need to be a Alcatraz once you know what you're doing enough that you can create more unit gains outside of the Alcatraz than what you're losing anyway.