Im very late to this discussion, but since I'm a historian and Second World War has always been one of my areas of interest I think I can contribute with sone information. Also, Im still coming down from my last read of the series
1. Im going to start with the most important thing in my eyes Tanks need certain prerequisites, especially second world war tanks. And I dont mean points you have already risen like engine, tracks, etc.
Tanks need roads. For armored warfare to be a reality, they need good asphalt roads. Yes they can deploy, attack and move cross country, but most of the time, whenever possible, they use roads that can take their weight. In other words, they go cross country strictly when they have to, and for good reasons: its faster, it reduces fuel consumption, and avoids the way higher wear on the machinery. I dont remember any description of Safeholds roads that would give me grounds to think they are up for mechanized warfare.
They also need a bunch of supporting weapons and services, mostly I was thinking we have not seen even one machinegun on Safehold, and tanks without their coaxial, driver and top machine guns would be of very limited use. There are more examples but those two were at the top of my head. Its not just the vehicle.
2. Steam engines should be a no go, and mostly for the same underlying reason you need radios. Doctrine. Tanks are versatile and mobile, a mechanized unit can move and react very quickñy. A steam engine needs to build up pressure before giving the required power. While Im uncertain how long that could be, Im certain it would not be the instantanous avalaibility of a combustion engine. The use of the tanks then would have to be under very safe parameters so as not to have inmobilized units at the worst possible moment. Usefull in set pieces situations probably, but not tank warfare.
3. Radios, or an equivalent comunication system, are a must for mechanized warfare. Theres plentiful historical evidence in this regard: it was simply not possible to coordinate units in anything resembling mechanized operations without it. To cite a few, from the Hundred Days Offensive in World War I it was observed that tanks operated better when there was a clear plan and objectives beforehand, but that once those objectives were achieved or the conditions of the operation changed tanks reacted poorly, in a disjointed fashion or not at all.
Closer in time, the infantry tanks attached to the french units have been mentioned here. Without radios, they simply could not either learn the information in time or react as a unit, even when numerically they should have had even numbers.
And a bit further in time, most soviet tank units in 1940-1941 did not have radio (and even later a doctrine to make best use of it) and it proved a major obstacle in the opening stages of Barbarossa, when the sovier command (Stavka) issued orders for counterattacks that were simply beyond the coordination possible for its units.
In summary, without communications you may have tanks, but not tank warfare.
4. Technically, and this is a very important point since most of what we are discussing or know about the topic is known too to the Inner Circle in splendid detail, I dont think Charis is far away from being able to design and produce a tank. Not a late war model like the Panzer V "Panther" (that yes was a disaster in 1943 and one of the best tanks of the war from 1944 onwards, after all it was design in less than a year so its not surprising that initially it was bug ridden) but IF diesel can be used there are excellent designs from the 30s, like the Panzer IV,the soviet BT-7 or the Czech 35t. With their real world data truly refine a design without any of their shortcomings.
5. Tanks are a force multiplier, and as such it makes sense for Charis to develop them and the doctrine that goes with them. They save lives and allow a small, highly ttained force to punch really above their weight. If you add the technology demonstation angle, its quite the idea.
Think thats enough
