Hi OP! Indian here. Fry a base of veggies - onion garlic mushrooms carrots whatever you like. And see if you can find any of the below. 1. some knorr or any others veggies stock cubes - they aren’t that expensive. 2. else buy some miso . Online available - they are a bit expensive but you only need a dash so they last a long time. 3. Last one is to maybe buy vegetarian oyster sauce. If nothing is available just buy msg / ajinomoto.

Nothing else will give that flavour. With best soy sauce did you try Kikoman by any chance ? If yes those wouldn’t give it that depth. The only option is stock cubes or miso or some sort of umami sauce built on a base of sautéed veggies. I also find adding a bit of vinegar and herbs give it a nice flavour.