Unfortunately, we don't have sources written by Norse peoples during the Viking Age, so answering this kind of question is fairly hard. The sources we do have are either 1) written by the victims of Viking raids or 2) written centuries after the fact, largely in Iceland.

That being said, we aren't entirely SOL here. Based off the sourcing we do have, I can say with some confidence that while enslaved people were treated fairly horribly in the Viking Age, they were not regarded as "subhuman" in any sense of the term.

The first reason I say this comes from contemporary Early English sources (I use that term instead of Anglo-Saxon because of the latter's centuries-old association with white supremacy). In these sources, underneath the biases, we can see a continuum between payment in silver, in goods, in people, or in violence. Slavery is explicitly mentioned in the 942 entry of the so-called "Anglo-Saxon Chronicle" -

Here Edmund king, of Angles lord, protector of friends, author and framer of direful deeds. o'erran with speed the Mercian land. whete'er the course of Whitwell-spring, or Humber deep, The broad brim-stream, divides five towns. Leicester and Lincoln. Nottingham and Stamford, and Derby eke. In thraldom long to Norman Danes they bowed through need, and dragged the chains of heathen men; till, to his glory, great Edward's heir, Edmund the king, refuge of warriors, their fetters broke. (translation James Ingram).

This is probably somewhat metaphorical - not all the Mercian people were enslaved! However, slavery was a prominent part of the Viking age, being as core as treasure. The poem The Battle of Maldon does not ever mention slavery, but the threat is implicit in the negotiations before the fight - give away your gold now and we'll leave, or we will kill you and take gold and slaves at our leisure.

However, slavery was not unique to the Vikings in this period - the various early English kingdoms all also kept slaves! A good overview of the evidence here is "Slave raiding and slave trading in early England" by David Pelteret, but in short, it was perceived of as a normal part of society, and not something that was inherently subhuman. This makes sense - there were long-lasting settlements in the Danelaw, and an Anglo-Scandinavian community formed that was popular enough that the St. Brice's Day Massacre - the 1002 killing of Danes in England ordered by Æthelred II unræd - was perceived of as a great evil.

The second point here comes from the eddic poem Rígsþula. In the poem, the god Heimdallr in disguise comes to three families, and gives birth to the 3 legal classes of Norse society - þræll, karl, and jarl (slave, freeman, and lord). In the poem, physical beauty and class are tied together -

Hann nam at vaxa
ok vel dafna;
var þar á höndum
hrokkit skinn,
kropnir knúar,
fingr digrir,
fúlligt andlit,
lotr hryggr,
langir hælar.

[He begins to grow
and thrives well;
There was on his hands
wrinkled skin,
swollen knuckles,
thick fingers,
an ugly face,
a crooked back,
long heels]

This is about as close to "subhuman" as the Norse corpus ever gets. However, this person is still able to be happy, and find love, and his descendants become the various manual laborers of Norse society. Obviously, this is problematic- it sterilizes how cruel Norse slavery was (slaves in other sources could be assaulted, sacrificed, raped, ordered to commit murder, and made to work in extremely harsh weather conditions without good clothes), and it makes it seem that slavery is an innate trait. However, given that Þræll can be happy, and that Rígr is his father using exactly the same language as for Karl's birth and Jarl's, it does not seem that this physical deformity is perceived of as being subhuman.

Finally, as you note, slaves (or their children) could be freed and achieve high rank. Pelteret notes that the Guthfrith, a king of Northumbria, may have been "originally the slave of a widow to whom he had sold by the Danes." This is about as unequivocal of a rise as you could ask for. However, freemen show up all across the later Icelandic sagas, sometimes in positions of great respect in a family, as Þorðr, the son of a freed slave and foster-father of the sons of Njáll Þorgeirsson, attests. Þorðr dies in a highly stylized feud, but the "equivalent" payment is killing a direct family member to the head of the other household.

The most dramatic example of the heights to which slaves could rise, though, is the Irish slave Melkorka in Laxdæla saga. Melkorka was the daughter of the king of Ireland, who was bought by Höskuldr Dala-Kollsson. She pretends to be mute, and he has a child by her anyway (which is creepy). This child, Óláfr pái, becomes one of the most famous and wealthy people in Iceland, and is the father and foster-father of the tragic next generation of saga characters.

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So, from these, it should hopefully be clear that slaves were not treated well by any metric of the term. They were taken in terribly violent conditions, a trauma that accounts from enslaved people in the 19th century indicates will never fade, and treated very badly. However, in practice, there was absolutely room for upward mobility and ransom from slavery, and overall no indication to think that Norse peoples viewed their enslaved early English, Irish, Slavic, and Frankish peoples as subhuman.