MMH5.5 - Heroes of Might and Magic 5.5

I will add that Nargot doesn't like long games and is unlikely to ever play with 5.5.

If the balance is tailored for normal timeframes as well (the first month of the game), I don't see a problem with this.
You need players who play efficiently and quickly to find holes in the balance, even if the mod supports long games.

Regarding spells. I won't go into long explanations, just ideas.
Orientation towards a prolonged game (2-3 months or more), therefore, higher-tier spells should be expensive.
While mana is scarce, one can play with low-level ones.

Buff/Debuff Durations:
Current formula 1.5/2/2/2 + 0.1*SP
Proposed: 1 + 0.2*SP

Purpose: to increase dependence on SP.
Mana regeneration speed: x2 (2 for each unit of Knowledge), to raise the value of Knowledge and compensate for expensive spells when clearing neutrals.

LIGHT

1) Divine Strength: 50/65/80/100% (5 mana) - ok
(mass effect on all troops)

1) Haste: 10/15/20/25% + 0.33*SP (6 mana)
Proposal: 10/15/20/25% (5 mana)
Why: dependence on SP is not needed here; this is an analogue of Divine Strength, but it boosts damage via initiative.
(mass effect on all troops)

2) Endurance: 4/4/6/8 + 0.1/0.15/0.15/0.15*SP (6 mana)
Proposal: (SP + 20) * 0.2/0.3/0.4/0.5 (10 mana) or the same 4/6/8/10 + 0.2/0.3/0.4/0.5 * SP
(mass version of the spell - 4x4)

2) Deflect Missile: 40/45/50/55% +0.5*SP (7 mana, level 3)
Proposal: 40/50/60/70% (10 mana, level 2)
(mass version of the spell - 4x4)

3) Cleansing: 40/50/70/90% + 0.1*SP (10 mana, level 2)
Proposal: 40/60/80/100% (20 mana, level 3)
Why: the chance should not be tied to both SP and level, as it complicates calculations for the player.
(mass version of the spell - 4x4)

Why Cleansing is higher than Deflect Missile:
- universal spell
- key (indispensable) spell against Darkness, allowing heroes with low SP to neutralize expensive and powerful spells of heroes with high SP
- allows removing several spells from a target/targets simultaneously

3) Righteous Might: 4/4/6/8 + 0.1/0.15/0.15/0.15*SP (6 mana)
Proposal: (SP + 20) * 0.2/0.3/0.4/0.5 (20 mana) or the same 4/6/8/10 + 0.2/0.3/0.4/0.5 * SP
(mass version of the spell - 4x4)
Why it is more expensive than Endurance:
- the spell is of a higher level (i.e., even the original game developers understood it is stronger)
- the single-target version boosts the active rather than the passive target
- the single-target version boosts shooters very well
- key spell for heroes who kill with troops

4) Regeneration: 30/40/50/60% + 7/8/9/10*SP, duration 2+0.1*SP (8 mana, level 3)
Proposal: 0% + 25*SP, duration 1/2/3/4 rounds (40 mana, level 4)

4) Magical Immunity: 2/3/4/5 (16 mana)
Proposal: can target both allies and enemies (40 mana)

4) Teleportation (10 mana)
Proposal: 40 mana

5) Word of Light: (SP+10) * 8/10/12/14 (16 mana, level 4)
Proposal: 10/15/20/30 * SP (80 mana, level 5)
Note: similar spells should be very expensive so they are not cast every turn.

5) Resurrection: 90/120/150/180 + 20/25/30/40*SP, HP reduction by 16%-0.2*SP (20 mana)
Proposal: 0 + 30/45/60/90 * SP, without HP reduction (80 mana)

5) Divine Vengeance: (SP + 10/20/30/40) * 1.1/1.2/1.3/1.4 * F (18 mana)
Proposal: 0 + 60/90/120/180 * F (80 mana)
Note: similar spells should be very expensive so they are not cast every turn.

General Note №1: skillful dampening of Light should work well for the entire level 5 (since that is its target area)!
General Note №2: Each circle has spells that are weakly dependent on (or independent of) SP (for roles of heroes with low SP but high Knowledge).
General Note №3: Endurance and Righteous Might are excellent spells for an army if there is a lot of SP (together they convert SP to attack and defense at a 1:1 rate).
General Note №4: heroes with 10% Knowledge (including classic Might) will not be able to actively use level 4-5 spells due to the prohibitive cost.

I won't detail other schools, as the time might be wasted.
Either you like this concept (using the Light school as an example), or you don't.

Added after 43 minutes

The original upgrades were very similar. The one with firebreath was preferred so we decided to make the second one a little slower and little tougher (more defense and HP) so it can be an alternative for Mages who want more endurance in their army.

Reducing the speed of the dogs is an unsuccessful change, and for good reason it concerns many players.
Speed is a parameter that should only be touched as a last resort, because it fundamentally affects the nature of a unit.

Dogs need speed to block shooters in a single turn during a push. If you want to diverge the upgrades, it's better to take away the retaliation attack (for the sake of increased damage). This would open options to absorb retaliatory strikes with other troops, as well as absorb retaliatory strikes with summoned dogs.
Increasing the HP of the dogs does not correlate with the role they play on the battlefield.