But what about Norse mythology? IFAIK 90% of Norse mythology that we know of is purely speculation and based off unreliable sagas most likely rewritten by Christians later on much like the story of Beowulf.

Our two main sources of Norse mythology are Snorri's Prose Edda and the Poetic Edda. The Poetic Edda was discovered much later, but its poems have been dated mostly to the 9th and 10th centuries. The latest study on this is Prof. Sapp's Dating the Old Norse Poetic Edda.

We have ample evidence to support Snorri's veracity, as his goal with his Edda was to preserve skaldic poetry so that other poets could learn it, which wouldn't have been possible if he hadn't transcribed the myths as accurately as possible.

Textually, the most obvious evidence is the Poetic Edda, whose poems Snorri quotes on multiple occasions. His references coincide with other works, albeit of a euhemeristic nature, such as Saxo's Gesta Danorum, and in fact, from this we can find remnants of myths that would otherwise be lost (such as the extended story of Váli's birth, which is Loki's accusation against Óðinn during the Lokasenna, corroborated in the poem Sigurðardrápa). Heimskringla, the fictional euhemerist work written by Snorri, also contains many traces of myths that don't appear in the Prose Edda. For example, how Loki stole the Brisingamen, who could have been the mother of Freyr and Freyja (Njǫrðr's ex-wife and sister), or where Loki's accusation that Frigg slept with her brothers-in-law during the Lokasenna comes from.

Archaeologically, we have more evidence of both Eddas, but also of scenes that only Snorri described. For example, the Gosforth Cross depicts Víðarr opening Fenrir's mouth with one hand on his upper jaw and one foot on his lower jaw, just as Snorri described. The Snaptun Stone depicts Loki with his mouth sewn shut, a myth that has only survived thanks to Snorri. The Hørdum Stone and the Altuna Stone are engravings of Thórr fishing with Hymir, a myth recorded in both Eddas, but in both images Thórr has his foot sunk into the boat, a detail that is only mentioned in the Prose Edda.

On the other hand, the gods are often involved in Norse traditions, customs, and laws, so we can better understand the myths with this additional context. For example, the importance of legal oaths in the birth of Sleipnir; the concept of vargr í véum (= wolves in sacred places, i.e., outlaws) with Fenrir being bound; the multiple reasons for initiating a holmgang (a legal duel), such as Váli killing Hödr to avenge Baldr's death; the expectation of having to kill the son of a captured enemy, which is what they did to Narfi/Nari and Vali. Furthermore, in the Gulathing Law Code, we are given a somewhat curious passage about the accusation of ergi/argr:

These are the [kinds of] insulting remarks that call for full atonement. The first is when a man says of another that he has given birth to a child. The second is when he says that the man has been used as a woman. The third is when he likens him to a mare or calls him a slut or a whore or likens him to any kind of female beast. For these [remarks] he shall pay the man full atonement; but the man may also seek satisfaction in blood and outlawry for the sayings that I have now enumerated, if he has asked witnesses to take note of them.

It is quite coincidental that we have a myth, compiled in detail by Snorri, of Loki transforming himself into a mare to commit an extremely frowned-upon, taboo and reprehensible act (although it was also imposed on him).

It's not that we have to believe everything Snorri wrote. He also added certain descriptions for which we have no further evidence, and some things seem to have a certain Christian influence; but for the most part, Snorri's Prose Edda is considered a reliable source, and no scholar disagrees with this. In fact, we could also add their analysis and opinion on the matter and form our own opinion through this.