Albums Like Nokturnal Mortum Voice of Steel

Well, you sort of named most of the stuff that could be recomended!

There are Lots of Pagan Black Metal bands but none have as much folklore influence as NM or Kroda.
the closest thing for me would be Hellveto from Poland. especially their In Arms of Kurpian Phantom and Medieval Scream and Stos albums.

another notable band is Darkestrah from Kyrgyzstan (all of their albums!)

another personal fav of mine is an Iranian band called Akvan (especially the 2017 Full-length and the 2018 Esfandiyar EP)

if you want the riffs of Voice of steel, there is nowhere better to look than Bathory's classic "Blood,Fire Death) or the first Skyforger album (From Latvia). or Ildra (England). if you like Graveland type of epic BM you should definetly check out Bilskirnir (Germany) (Especially mid 2000 LP's and EP's like Wotansvolk). or Nordreich (also from GermanY). also Poccolus from Lithuania.

if you like the same kind of Folk-y approach with less Riffage, i can reccomend other bands from Ukraine like; Dub Buk (the first album)
Astrofaes is another great Ukrainian band too (with a little thinner guitar tone!)
another one would be Melechesh from israel (Especially the Sphynx album)

also ARKONA from Russia (it's a bit far from NM but the folk-y parts have some resemblences)
Windir's Arntor (from norway) has the same Epic-ness with little less folk-y elements and instruments (again, a bit far off but still cool)

there are others too but i think we're getting a bit far from them! Graveland is the most similar (as yourself mentioned)

I've been out of touch with the folk/pagan circles for some time but Darkestrah certainly gets my vote too, unique band with a very underrated female vocalist called Kriegtalith, on par with Angela Gossow, although it seems she isn't with the band anymore.

One pagan band that i really recommend you and that i recommend to everyone who like pagan black metal is Wolfkrieg. My personal favorite release is Последняя весна (Last Springs in russian) EP and the album Another Battle but they are really hard to find even on online music platform but there is a compilation call Fatherland who contain song of the 2 release that i just mention and other song from the previous release that you can find on Bandcamp. My favorite pagan black metal band by far. Hope you enjoy.