I challenge you!

Upgrades shouldn't be random. For example, the difference between the upgrades for Mages, Archers, and Marksmen is significant. Either make it random only for those that are more or less equal.

Okay, but randomness would add a unique element and a sense of surprise...
Militia, Archers, Halberdiers, Core Gryphons, Adepts, Knights of Isabel, Archangels;
Imps, Greater Demons, Cerberi, Succubi, Hellhounds, Cave Lords, Archdevils;
Skeleton Archers, Plague Zombies, Ghosts, Vampire Lords, Archliches, Death Messengers, Astral Dragons;
Assassins, Furies, Minotaur Overseers, Dark Riders, Dark Hydras, Shadow Lords, Black Dragons;
Nymphs, Wind Dancers, Forest Archers, High Druids, Light Unicorns, Wild Ents, Crystal Dragons;
Gremlin Saboteurs, Obsidian Gargoyles, Obsidian Golems, Archmages, Djinn Viziers, Kshatria Rakshasa, Titans;
Warriors, Harpooners, Bear Lords, Arkat Warriors, Flame Priests, Jarls, Lava Dragons;
Trappers, Battle Centaurs, Warmongers, Daughters of the Earth, Executioners, Paoka, Free Cyclops.

Added 9 minutes later

As practice shows above, there are already many of them :(
Perhaps reduce levels 6 and 7, but add levels 5 and 4.

No. I think even with a 25% increase, not everyone has tried them all. Maybe that will be enough.