There are several occurrences of the word "spangabryjna" in the Old Norse sagas. In the Sverris Saga, Earl Erling is described as wearing it to the battle in which he's killed. In Hakkon Hakkonarson's Saga in the Heimskringla, Gregorius survives a wound because he is wearing spangabryjna. In the Laxdaela Saga, Helgi describes an enemy as wearing spangabrynja.
All three of these sagas, and throughout other sagas, armor is continually described as "brynja", rather than "spangabrynja", except in these three cases, written several decades apart in the early 1200s.
I've seen theories ranging from lamellar, scale armor, or early coats of plate, but there's no archaeological evidence of any of these.
In the view of expert historians, what might the term spangabrynja refer to?
I, like you, have seen theories on lamellar, scale or early coat of plates, and even more perishable armour, like woollen gambesons or leather armour (or even leather lamellar armour). As far as I know the debate is still out there.
However, there are archeological evidence of viking-era lamellar armour. Finds are scant, and the debate on wether it was locally produced or traded for in contact with the Rus and their steppe nomad or semi-nomad neighbours is still ongoing.
The Snäckgärde site on Gotland was unfortunately excavated before the advent of serious archeology, and we only have written records of it. One of the men buried had
några pansarfjäll
"Some armour scales", which most seem to think were lamelles.
The hall at the viking-era trading town of Birka outside Stockholm, usually called "the garrison" for the large amount of weapon and armour related finds below it has produced more than 700 lamelles of which 8 different types have been identified. However, some such as Dawson theorise that these were fragments recycled for other use, or captured pieces that were used with other things (like spearheads) to decorate the hall with trophies.
Then we have the find in the Korsbetningen mass graves at Visby, where a Gotlander peasant militia was defeated by King Valdemar Atterdag's force of Danish knights, men-at-arms and mercenaries. At last two lamellar armours were found in the graves, one in remarkably good condition. The Gotlanders were defeated 1361, but the well-preserved armour shows signs of having been recently modified - it had been riveted to a leather "apron", which would make it look a lot like the coat-of-plates that a lot of the other men in the graves wore. We can't know for sure when the Gotlander lamellar armours were made, but they have been theorised as old armour brought out in a time of crisis, hastily modified to look like modern armour (including for the time modern shoulder armour) and used in the battle.
The Birka finds show that the viking era Swedes at least knew of lamellar armour, as they had quite a bit of it in their "garrison" hall and while it is possible it was there as decoration and trophies, I am inclined to believe those that argue that at least eastern Scandinavians around Svealand and Gotland knew of and to a limited extent used lamellar armour - either imported or locally made and that "spangabrynja" then probably would refer to lamellar armour. But that is my personal opinion as an amateur. Further reading and details and in the future hopefully new archeological finds may shed more light on this issue.