Ideas and suggestions.

I didn't quite understand what functionality you're discussing. There are already many of these suggested, so it would be good to clarify.

For example, I didn't understand this phrase:
Does it not work at all when based on a spell?

I don't think I said anything or agreed with the statement about "not working/only transferring from itself when based on Town Portal troops."
To summarize:

For Mana regeneration (level 5), I think it's better to give a fixed number instead of a percentage, so that regeneration doesn't depend on logistics. The numbers could be:
* +250 without Water Mastery
* +500 Basic Water Mastery
* +750 Advanced Water Mastery
* +1000 Expert Water Mastery
The cost in mana is similar to other control spells - 20 mana without Water Mastery. + a limit of 2-3 times per day per hero.

Creature Teleport (level 3) = teleport creatures from the casting hero to a selected one (selection window like with Expert Town Portal) without range or hero selection restrictions (similar to the base Town Portal). That is, you can always choose who to transfer troops to. Costs 12 mana without Fire Mastery.

Dependence, in my opinion, is better based on Skills Points:
Creature level: without school / basic / advanced / expert average increase
1st: 1sp 8 / 3sp 32 / 1sp 16 / 1sp 24 16
2nd: 1sp 6 / 1sp 8 / 1sp 12 / 1sp 18 12
3rd: 1sp 4 / 3sp 16 / 1sp 8 / 1sp 12 8
4th: 1sp 2 / 3sp 8 / 1sp 4 / 1sp 6 4
5th: 2sp 3 / 1sp 1 / 1sp 3 / 2sp 9 3
6th: 1sp 1 / 3sp 4 / 1sp 2 / 1sp 3 2
7th: 2sp 1 / 3sp 2 / 1sp 1 / 2sp 3 1
For refinement, you can make a coefficient for each unit separately.

Rounding is always up, i.e., with 2sp and Basic Fire Mastery, the hero will only be able to teleport 1 unit of level 7 at a time, because 3sp are needed for 2 creatures. In fact, this is a mana limit, since the number of times is not limited. So, maybe it makes sense to limit the number of troop teleports per day per hero as well?

In general, troop teleportation, in my opinion, is a great idea. In addition to speeding up games, you can get leveled-up heroes + you can get a bonus to Necromancy, which was heavily nerfed in HotA + you can get games in which, in addition to the Fortress Battle, there are other encounters (add new templates for this, maybe?).

For newbies, it's a must-have, so the cost of Fire Mastery increases significantly.