Gil-Galad w/ armored horse and shield
Elrond w/ heavy armour and horse
Glorfindel, Lord of the West w/ Armour of Gondolin and Asfaloth
Cirdan
Elendil w/ horse and shield
Isildur w/ horse
Thranduil, King of the Woodland Realm

Planning to be a really combat-oriented list, with heroes that can not only reliably survive, but flourish in the midst of combat for many turns. Any thoughts?

Firstly, everyone needs to be mounted, so give Thranduil a horse as you can always dismount him once you get to combat, it is easily still worth those points.

Gil-Galad, Glorfindel and Elendil are great choices. Islidur is alright but cannot use the ring on horse.

Elrond is a bit overpriced for me, and I would swap him for Aragorn with armour on a horse as that free point of might will be so much more useful for heroic moves, combats and strikes (he could be at least fight 7 and likely higher every turn with striking).

I am guessing Cirdan was for blinding light? But with a 6" move and a fellbeast that could easily sap will him, he is a waste in this force as he cannot keep up. If you really want shooting protection then Gandalf on horse is the only way in my opinion, and you would probably include legolas on horse as well, but you said you wanted combat so I wouldn't bother wasting those points.

Other heroes I would consider are fellowship Boromir (those 6 might points means he will do a lot of damage before he goes down). Just be reckless and use those might points fast and he will be worth it for his cheap points. The Twins are also really nice for their points cost and give you two heroes with 3 might and 3 attacks for a good price. I know there aren't models for them yet, but Dwalin and Thorin on goat are absolute beasts (Dain is good too) and if I had models for them I would certainly consider them although the 8" move is not great. Beorn is also worth considering as that gives you hurl, but his 8" move and needing to change are not ideal in an all hero army needing to get to combat fast. Imrahil and expensive Boromir have lances and are not bad but I think there are better.

My list would be

Glorfindel w/Asfaloth and armour
Gil Galad w/armoured horse and shield
Elendil w/horse and shield
Aragorn w/horse and armour
Thranduil, King of Woodland Realm w/ horse
The Twins w/horses and heavy armour

or in a fantasy world and had all these models go for an 8" move team with survival mounts and just march with Boromir

Glorfindel w/Asfaloth and armour
Gil Galad w/armoured horse and shield
Boromir w/horse
Thorin w/goat
Dwalin w/goat
Thranduil, King of Woodland Realm w/elk and bow (spare points)
Dain w/goat

With getting to combat I would put the less valuable horses (twins, Thranduil) at the front but use scenery as much as possible to avoid being hit. I would also consider heroic marching as well, especially with someone like Boromir as that will reduce the time it takes to get to combat.

The list seems ok... It is thematically viable, and it is also a good choice to keep the eras separated!

A bit high points though, I hope you can find an opponent to play with this high points...