Heroes 5.5 WarGame Edition - Tactical Edition.

Nah, replacing knights with sunbeams just doesn't seem serious. Like, we don't have regular horses, or heroes ride them, but here, take sunbeams. It would be much better suited for another, more magical faction; for example, it would be really cool to bring back elven riders on pegasi. But no one has made such models, and in that case, we would have to give up the equally iconic unicorns (or druids).

I agree, that's why I wrote

...There are similar units in the lore, but they don't fit here - I'm talking about the "radiance" and "sun riders" units...

And I can't really think of anything in terms of who could be placed in the Humans' 6th tier - because, logically, there shouldn't be humanoid units in the 6th tier, but griffins would be too powerful there. Again, there's a rather wild option of lowering angels to the 6th tier (the logic is that, according to the lore, these are not angels at all, but a product of the fusion of the souls of a deceased person and an angel - that is, an intermediate form), but then who to put in the 7th tier is unclear, and this is too creative a shift to move the winged creatures from the 7th to the 6th tier - a wild option.
In general, we come to the conclusion that it will not work to equate the strength of creatures of one faction to the strength of creatures of another faction, and it is possible to balance them by increasing their stats and cost. Well, for example, like this - what is the 7th tier for humans, is only the 6th tier for warlocks - the question is in the stat increases and cost.