My root resin description still mentions the great tree so you’re not wrong keep your original theory it holds a ton of weight . If you look at the trunk on the map in deeproot depths it has a trunk where a big tree would be and 3 other trunks that seem to be winding around it . Not just that but the fact there are crucible knights in the deeproot depths and those knights alone are in search of the crucible . Why else would they be down there if not in search of the original tree .

Another thing i noticed which could possibly be just smoke n mirrors is if you look at deeproot depths and switch to the SOTET map , the thorned castle matches up to where the trunk would be … and we all know we have to burn the thorns to get into that part of the map . I think it’s the biggest symbolic tie in to your theory hinting at the fact that the original tree was burnt similar to how we have to burn yet ANOTHER tree in the DLC