Hmm, kinda embarrassed at some of my earlier posts, made after just one viewing...
The miniseries did handle Holly better than the other adaptations (especially the 1999 series, where he doesn't have a single spoken line between Cowslip's warren and the final episode

). The relationship with Hyzenthlay could have had more buildup, but they do share one cute moment...just before the tragic death. The death was rather pointless, since he survived in the book, but it fits with the pattern of each adaptation killing one good guy near the end...
"You Lack Animality! You have no integrity. Animals don't behave as like most men. If they have to fight, they fight. If they have to kill, they kill, but they don't devise ways of hurting other creatures." - Captain Holly Watership Down (2018)
Perhaps the most awesome moment, when he stood up to Captain Vervain in Efrafa
"What makes a good soldier is not always what makes a good rabbit." - Captain Holly Watership Down (2018)
The very most important moment for me when he scolded Captain Campion over the atrocity of rabbits being marked. Very relevant about how soldiers are being looked at in the human world, past and present.
The first quote is very similar to one in the book, although in the book it was Strawberry who said it

I think it fits in well with miniseries!Holly too though.
The soldier one quote is also pretty great; this one appears to be entirely new.
He may be the only rabbit who meets all the qualities to be a substitute leader after Hazel.
In the book, it is said that he was considered to become Chief after Hazel was presumed dead at Nuthanger Farm. However, he said that he was in no condition to take it on (considering the disastrous Efrafan visit) and it ultimately proved unnecessary since Hazel wasn't dead

He probably could have done a good job, though.