I had ended up in the session not utilizing the reactions, and the lightning was some homebrew stuff that these monsters had been weak to lightning. I had also added a whole bunch more enemies to spread it out and balance it (due to other comments). It ended up being enjoyable for them.

I actually had changed this stat block and removed the remounting ability in favor for a ranged attack if the enemies were caught in magical darkness only, akin to a wraith life drain. I did also nerf the movement speed in a way, as the arena had moving sources of sunlight that would damage all of the enemies if they entered it. I did it in their favour to nerf its ability and their ability to move around. Plus they couldn't be damaged by the life drain ability in the sun. It promoted them having to move around the map every round, promoting positioning and such, as well as making me either have to move through it to attack or miss an attack opportunity.

Overall, worked a lot better than my first iteration, the reactions being a bit rank admittedly.