Heroes 5.5 WarGame Edition - Tactical Edition.

Currently, artillery/sulfur rain differ slightly, but their combined effect is the same:

Artillery: Before each battle, up to 20% of large enemy units are killed (rounded down).
Sulfur Rain: Before each battle, up to 20% of small enemy creatures are killed (rounded down).

As rightly noted, 20% is a lot, but the rounding is done downwards, which means that tier 6 units will often either not be killed at all (except for neutrals) or will be killed in a noticeably smaller number due to rounding (e.g., out of 9, only 1).
In addition, the details are not described, but it is implied that it is possible to counter artillery perks by splitting up troops. That is, the 20% effect will be applied to each stack; for example, 2 stacks of 4+4 wyverns will not be affected at all.
On average, it is assumed that the actual losses will be around 15% (due to rounding). And that's not that much; the same skilled defense + evasion (4 perks) gives a -30% reduction in physical damage from enemy creatures. Here, advanced engineering + artillery and sulfur rain (4 perks) will give a -15% reduction in enemy creatures.

Overall, engineering should work better than in the original. Since firearms are not necessarily required, and a three-shot ballista without firearms (which cannot be taken at the same time) will work approximately the same as a three-shot ballista with firearms in the original, i.e., the same effect for a significantly smaller number of perks. The tent will also work much more effectively (25/50/75/100). And the artillery branch should not lag behind either the ballista or the tent.

The number of "30 elite units" by the time of the endgame will rather refer to tier 3 than to tier 6, unless the game is specifically prolonged. :)
The concept implies a game in the spirit of Starcraft, with early interaction between players and early encounters, rather than preliminary isolation for a month or two while waiting for a giant army to be assembled.

and WGE is designed for a game duration of approximately one month.

Within 1 month, but not a whole month. The ideal average time for "final" encounters is 2-3 weeks. In chess terms, week 1 is the opening, week 2 is the middlegame, and week 3 is the endgame.