To answer your initial question “is there something that can be done outside of remaking this boss”:
No, there is not.
A 1800x900 size sprite is 6.5 mb of VRAM in a GPU (textures are uncompressed when loaded into a GPU, so the memory size would be length * width * RGBA (4 pixels for color and alpha)). That’s for a single frame. at 600 frames for a single animation, that’s 3.9 gigabytes for your animation to be stored in your GPU’s ram.
For context, Hollow Knight’s largest single sprite size is 500x500, with the screen size ones broken up into separate parts. This allows for the parts not currently being rendered to more easily be culled from memory. Also, most of Hollow Knight’s bosses have most of their parts as non-animated single frames that are moved with game logic, so there’s no animation frames being loaded.
Edit: Your previous posts mentioned you were using Skul as a reference/comparison. I did some digging and Skul’s native resolution is 640x360 and their largest sprite has a max height of 400. Your assets, unfortunately, may just be too large for your GPU (or most game engines). Scale and create captivating 2D pixel art | Unity Blog | Unity Blog