Heroes 5.5 WarGame Edition - Tactical Edition.

Well, I've already outlined the optimal progression for myself:
Week 1 - Opening (in principle, you can develop to any technology, for example, to Tier 7)
Week 2 - Middlegame (maximum progress, you can build all the important structures for the chosen strategy and key upgrades)
Week 3 - Endgame (development is coming to an end, secondary buildings are completed based on remaining resources)

That is, if the game hasn't ended with a rush in the first week, then weeks 2-3 are a reasonable timeframe. After that, it's for those who enjoy long games and want to clear the entire map.
The reason for this is that 3 weeks is the minimum (fastest) time in which you can play through all 3 stages with clear distinctions. Since at the beginning of each week, creature growth is updated (many new creatures become available for recruitment), this is a natural and convenient boundary between the stages.

Regarding neutral castles, is it possible to make it so that one building is constructed in them each turn, and troops are automatically recruited when they appear? If not, the idea of building a couple of structures in them (the further away from the main castles, the more buildings are constructed) isn't that bad.

I've thought about this, but it would look strange for strategies where a player intentionally delays capturing a city to get extra buildings for free, which would accumulate in the city every day.

Tier 7 on day 4 is really detrimental to diversity and speeds up the game's pace too quickly ("from rags to riches").

Currently, the plan is to have Tier 7 available on days 5-7 (depending on the Faction). The placement of Tier 7 on day 8 for one or more Factions is still under consideration. This would mean that developing to Tier 7 for these Factions would be impossible in the first week (like in Heroes 3 before the Armageddon's Blade expansion, if there is no Tier 2 starting dwelling).