In my opinion the tomb kings shouldn’t have much of a tech tree. After all, none of their warriors are ‘trained’ as such. There could be workshops for the production of siege engines, and basic buildings, basically ‘armouries’, which allow progressive sets on units. For example, with the first the worst units can be made, to represent low production capability, working up until you can make fine armour and weapons for units like Tomb Guard and Ushabti.

After all, it isn’t as if they’re advancing. They died millennia ago!

As for the chariot idea of stretching an elephant… That made me laugh. Funny image of a stretched elephant running about.