Heroes 5.5 WarGame Edition - Tactical Edition.

It's illogical because the actual number of losses already increases over time (stronger armies appear). A double increase isn't needed here, especially since an increase with level means a weak start and a mega-imba in the finale (otherwise, weakening), which is exactly what I wouldn't want.

The balance inevitably changes in the late game. This is a given. With a base movement range of 40 (instead of 25), map size isn't a problem.

2 months isn't relevant in competitive Multiplayer, even in the original game. Okay, let's consider these timeframes for single-player (does this mean that in the original, the same map would take 3 months to complete?!). I assume that inevitable losses on neutral creatures weren't taken into account at all? So, the game is completed very slowly in a light mode "without losses"? Okay, in that case, in the late game, lower-tier units don't play a role compared to top-tier units, which have a 3-4 times higher growth rate. Thus, we don't get such a significant bonus (weak units only catch up to strong ones).

Also, your calculations assume that this perk is taken from the very beginning, and not later. This means, among other things, that early-game breakthroughs suffer (when there's no benefit from it, just like there's not much benefit from the Leadership base). This will inevitably affect the late game, like a snowball effect.

You also didn't take into account troop replenishment from other cities (is a 2-month map played with only 1 city? I played in Heroes 3 (with similarly boosted logistics) in 2-3 weeks, already with 8 cities, 2-3 of which were native). So, the relative bonus from Gathering for lower-tier units will be far from 300% even after 2 months, but much less.

That is, your calculations are made for: a huge map, for some reason with a small number of cities, and also for a slow pace of gameplay. This is a combination of too many conditions at once. And yes, I understand that maps for single-player in Heroes 5 are usually designed this way. But I can't play them myself, and therefore I also can't balance the mod for a game mode that is foreign to me. I don't understand why you would continue the game after fully building a city (including even unnecessary buildings for the chosen faction) and achieving key upgrades (without excesses), and also why you would keep dozens of dragons in your army and complain about weak Chaos.

The Necromancer gets more bonuses from this perk (due to the combination with Necromancy). But that's logical, because he doesn't get anything from the Leadership base in terms of breakthrough, unless he mixes units. Also, looking ahead, Necromancy will give 5/10/15/20%, and not 20/30/40/50%, which will not have a significant impact on the total number of raised creatures (due to limitations on magic power), but it will affect the composition; it will be much more difficult to recruit only lower-tier units and, accordingly, boost Troop Gathering.