In the seventh game, I was impressed by the fact that each race has its own branch of abilities in the skill wheel.

The fifth game is no worse in this regard. There are three levels of racial skills, three abilities for each racial skill, and a special racial ability, which in most cases is very powerful compared to ordinary abilities. It is also called a super ability, but it has strict requirements for its development.
Additionally, there are tricky connections in the skill wheel (in the seventh game, this is implemented more simply), some special abilities that some races have in their wheels, while others do not, and a radically different wheel for the Orcs, combined with unconventional magic.

Some say that in the seventh game, units have few abilities, but I didn't notice that. It's just that most of the abilities are not active, but passive, and each unit has these passive abilities, from one to as many as five. This also provides a lot of room for maneuver and makes the gameplay interesting.

The same applies here. Only, I personally don't think that the seventh game has a problem with the number of abilities for units. Rather, they have a very pronounced "class system," which limits growth/stats. In the fifth game, this is better, in my humble opinion.

The only thing I noticed that makes the seventh game less interesting than the fifth is the lack of alternative upgrades - again, this would add more variety.

Alternative upgrades are quite cool, although they are often not properly balanced.

the fullness of 3D (how wonderfully the maps are made there!)

The maps are also good in the fifth game. However, judging by all appearances, you will appreciate how the interiors of cities are made in the fifth game. You won't find such impressive views in other parts of the Heroes series...

the built-in encyclopedia

Well, I can't say that I particularly liked the encyclopedia in the seventh game. To be honest, it's not very informative in my opinion. Unless you are interested in seeing a general picture of a large number of units, heroes, etc., at once, but the fifth game has an excellent fan-made guide for this.
If anything, I generally approve of the idea of built-in encyclopedias; it's just that in the case of the seventh game, I think it could have been done even better. Although, this is a matter of taste...