Amazing! I have a similar background.
I too was once upon a time leaning towards Advaita. I initially thought this idea of a Cosmic Consciousness where everyone has the same "I" is interesting. This Cosmic Consciousness is the Self (Atman) in Advaita, which is identical with Brahman, the unchanging substratum of the universe, the Transcendental Ultimate Reality. The Atman is an eternal indestructible core of one's being.
However, when I read the Buddha's explanation of consciousness being dependent on the six sense media, I realized that this is the only explanation that matched my experience. There was never a time when consciousness was cognizant of anything, except through the six sense media (eye and forms, ear and sounds, nose and aroma, tongue and taste, body and tactile sensations, intellect and ideas).
The Buddha made it very clear that consciousness is dependently originated and is impermanent. Just as a flowing river appearing as one stream, but in reality is constantly changing in content, similarly, consciousness too may appear like one constant permanent thing, when it isn't.
It is very clear that Buddhism and Advaita are completely far apart.
There's no Cosmic Consciousness in Buddhism. In all phenomena, there's no such thing as a self, as in something like an eternal core of being - in Buddhism. And therefore, no Eternal God. There is no Transcendental Ultimate Reality, which is a substratum for the cosmos in Buddhism. Everything is impermanent except Nirvana. Nirvana is not any kind of reality or self. Nirvana is simply the highest bliss and peace that is cognized with complete liberation from suffering.
There's more - the Bhagavad Gita 2.17 says that the eternal soul pervades the body while Buddhism says that all phenomena is not self, and the self is simply an impermanent mental idea that arises from the inter-operation of the five aggregates (form, feelings, perception, mental formations and consciousness). How they inter-operate is described by Dependent Origination.
This will be clear to you, if you dig deep into (what I consider to be) the Buddha's original teachings in the Sutta Pitaka, of the Pali Canon, which belongs to Theravada. Mahayana preserves these same teachings in the Sanskrit Agamas.
Mahayana, at first glance, appears to look more like Advaita than Theravada. But as I dug deeper into Mahayana, I found that it overdoses on metaphors and poetic language, making it appear to look like Advaita, but in reality it's not. For e.g. Mahayana has the Eternal Buddha that sounds like Eternal Brahman. But actually, this refers to the Buddha's Dharmakaya (body of teachings) that originates from his statement that when you see his teachings, you see him, and vice versa (i.e. pay attention to his teachings and not his form or personality, to know what's special about him). In my opinion, Mahayana is a victim of its own poetic genius, resulting in its historical disappearance from the land of its birth.
Now back to your question ...
Advaita seems similar to objective idealism, while Mahayana seems similar to subjective idealism, if we were to compare them to ideas in western philosophy. Both ideas are subset of idealism.
Is Mahayana really subjective idealism?
Nagarjuna taught that all things are empty of inherent essence or "own being" (svabhava) including emptiness itself. Somebody tried to say that just like people are empty of a self of people, the chair is empty of a self of chairs.
What does that mean? And how is it related to the mind?
In our minds, we have the idea of "I am the thinker" i.e. the idea of the self. That's the primary object in existence in our reality. We also have the idea of non-self objects i.e. everything else. We objectify and classify everything around us, into non-self objects, according to their relationship to the self. For e.g. my hand, my car, not my friend, not my country.
When you look at the waters of the sea from up close in a boat, you may feel fear and insecurity, especially if you don't know how to swim and have motion sickness. To the sailor, it's a source of joy and adventure. To the fisherman, it's a source of livelihood and he sees it like a mine or oil field. To fish deep in the sea that has never left the waters, the concept of water doesn't occur to it at all, as it does not know any other reality.
Another example - a piece of cooked meat appears like delicious food to the meat eater, and it appears repulsive to the vegan. To a honey bee, it appears like dirt because it's not its food.
These examples go to show that objects do not have the inherent essence given to it by the mind.
What's a body of water to me is nothing at all (or perhaps everything) to the fish. The waters of the great sea, as a place to sail and swim, and as a body of liquid, doesn't really exist, except in my mind. It certainly doesn't exist in that way to the fish.
What's delicious food to me, is dirt to the honey bee. So, the delicious food doesn't really exist, except in my mind. The dirt doesn't really exist, except in the honey bee's mind.
This concept is called papanca in Theravada, which is objectification plus classification, also known as reification. And it's related to anatta (the teaching that all phenomena is not self), because papanca is when non-self things are reified into objects and they are classified relative to the self. The idea of the self is also papanca.
So, in my opinion, Mahayana and Theravada are very much the same in spirit although different in form, while Advaita appears similar in form to Mahayana but is completely different in spirit.
