Spiral campaigns.

I played this campaign...
And overall, I was somewhat disappointed with the game.
The player accumulates strength, but then what?

I think that since the series has deteriorated, we need to introduce modifications. For example, at the beginning, give the hero not 0-0-0-0, but 0-0-5-5. Also, give larger and longer maps so that it is more interesting to play.
Suggest what else can be done.

So that there is no disappointment, it is necessary to maintain the game balance and keep it interesting...
I don't see any point in these spirals at all, but since you are making something like an RPG with a gradual progression, one enemy after another, then:
1) the difficulty should increase, i.e., at the beginning, you can come up with battles with a minimum number of units and defeat the designated neutrals with certain spells, then with smaller forces and a minimum of magic, move on to AI and various objects with non-fixed neutral squads, and then give the player cities, units, magic, and stronger enemies (or achieve difficulty with small squads and a minimum number of spells in the spellbook)
In particular:
!It was originally implemented on the Venegeance is mine map (we have Inferno troops and use these troops to defeat several AI with the same number of troops each)
!In Paragon, the start is excellently implemented in battles with peasants and swordsmen, and then along the way to the first castle, and throughout the game, the difficulty and possibilities gradually increase
!In Wayfarer, there is also an excellent implementation with initial battles and the need to carefully look at the paths on the map with gradual development
These are examples of different mapmaking options; in general, there are a huge number of options, and there are more than enough decent maps to see how certain ideas work.
2) Objects - the road should not be empty; the player should have something to do; otherwise, it will be extremely boring, and they will not want to continue. At the same time, there should not be too many objects, but not too few; there should also be logic.
3) Path - if it is just a spiral, then some kind of plot is essential (like the main character Orpheus going to the underworld to save Eurydice (and the condition for victory is to defeat her guard or capture the city and go through the circles of hell; something like that should be the plot, or you come up with it yourself, or based on existing literature, of which there are many), you can make some forks, dead ends, portals, and other things to speed up or diversify the game.
!Storylines are very well implemented in the trilogy (Titanic, Angelic, and Devil Pride); of course, you can simplify the game and go differently than the author planned, but in general, look at how the plot, quests, and hero movement are implemented; the passage is not particularly difficult, but it is engaging.
!You should not focus on making something like Meta-Serk (even naming the map correctly is difficult; there are no words to describe the passage; you have to look at and calculate every movement point, and in battles, sometimes calculate some crazy things, although the map is a masterpiece in its own way, but not for mass repeated playthroughs for fun).
4) AI actions - if everything is linear, it is better to make them static and adjust the balance so that there is interest in battles with them when the player arrives, or otherwise, think about their places of residence and possible actions - this is the most difficult.
5) Battles - they should be diverse; there should be at least the appearance of difficulty or something authorial; you can implement ideas with a tent, a ballista, and other things that are usually not used or are not popular; that is, there should be some kind of highlight and originality.
For example, look at the maps Le Dernier Elementaliste and Apocalypse by the French, where from the start, the main striking unit is fairy dragons; the battle is built around them; this is something original and unusual, and you need to introduce such things for the playability and interest in the map, and especially the campaign.
6) Quests and artifacts - it is essential that the player is not bored, does not just go through the motions, but is also forced to complete quests, receiving adequate rewards for it, and you also need to think about artifacts, where they will be, which ones you need to give and take, etc.
7) Test maps, work on the balance - play and play the map, editing and editing individual elements, achieving the desired result.

Regarding the spiral - look at the map Gift2001, it is partially there; there is no difficulty; the only loss is in the battle with the first AI, and then it is just simple and boring; there is no longer a need to visit anything, etc.
So, if you are doing it sequentially, you need to do it so that there are losses, a change in the types of units, and there is no sweeping away of everything in your path (although with AI, this is inevitable in the end; you need to artificially and skillfully strengthen it).

1. Passing with one hero and one unit until the mana runs out.
2. Passing with one hero and a small number of units after 2 weeks, when the opponent has developed a little, and one unit will not defeat him.
3. Passing with a pumped-up hero, sweeping away everything in your path.
4. Building magic towers to get the remaining missing high-level spells. It is tedious and takes a long time to accumulate resources to build towers.

You need to avoid this, so that there is no banal accumulation of meat and trampling everything in your path; perhaps the most striking example (in general, an anti-example - how not to do it) is Empire of Worlds; both the 1st and 2nd ones suffer greatly from this - we have a lot of troops, and in the end, everything is won easily; there are only a few worthwhile battles throughout the game.
Another example of an unsuccessful blunder is the Battle of the Titans (where you need to defeat Lord Haart with 4k ghosts to win); the map has a terrible imbalance; everything is stuffed without much meaning or logic; there are also uneven respawns; in some places, you really need to sit and skip, and in some places, there is just a lot of space; and the main enemies are completely locked up, and by the time you arrive, they do not represent anything, there is no interest.
The player should receive spells and resources in a timely and logical manner during the passage; it should not be the case that you sit in one place for a month, skip turns, then build, skip another month, buy, and in two weeks, clear the map.
And again, the player must receive the necessary spells, units, and artifacts for the passage 100%, but, of course, it should not be the case that we have several fairies, and all the spells are available, so that nothing else is needed except mana.
I hope the general idea is clear; in fact, it is difficult to make an interesting, replayable map.
Good luck!