Some thoughts I had on this,
While the Mahayana sutras do have a more expansive cosmology, it is it not necessarily that much different than that of the EBTs. It is a many worlds cosmology, with many realms and different types of beings, including many Buddhas. The idea that there may be other Buddhas (or highly spiritually developed beings) in other world systems is not emphasized in the EBTs, but it is not incompatible with it I think. I’ve always thought that it is also pretty compatible with our current view of the universe, especially if you accept any of the “many worlds” or “parallel universes” ideas out there.
What makes Mahayana distinct is the idea that these awakened beings in other realms are actually active in our world in some way. That, and the fact that Mahayana sutras include much more baroque and extensive expositions of their cosmology, and include way more miraculous events, whereas many EBTs do not emphasize this or even contain miracles. So Mahayana texts seem harder to believe than EBTs (which have less miracles and cosmological stuff) and some Mahayana sutras acknowledge this, saying how it is rarer for people to accept Mahayana and so on. However, Mahayana sutras also contain various ideas that give them a lot of hermeneutical flexibility, mainly the idea of upaya and the two truths (as well as a general critique of the use of language for explaining the ultimate reality). These ideas allow Mahayanist exegetes to interpret the sutras in different ways, and hence, like Patton said, you don’t need to take it literally (but also, you shouldn’t take anything literally since all reality is empty!).
Also, even though there many different interpretations of the EBTs, in Mahayana, this is turned up to eleven. There are extensive doctrinal and philosophical differences among the various Mahayana texts, they do not all agree with each other. So most Mahayana traditions have highly developed hermeneutics and philosophies in order to explain these seeming contradictions. So, with Mahayana, you are never expected to read the texts cold. Instead, one studies with a teacher, or least looks at the commentarial and philosophical tradition.
For example, the Prajnaparamita sutras are read together with the works of Nagarjuna and other Madhyamikas, as well as the Indian commentaries on the PP texts themselves (in Tibet: The Abhisamayalankara, in China: The Dazhidulun).
But anyways, I agree that one should first get a foundation in the basic Buddhist teachings before jumping into Mahayana stuff, since Mahayana texts assume knowledge of early Buddhist ideas, and then proceed to expand, critique, engage with or play with them.