You can use black bean sauce in place of the sweet bean paste. Black bean sauce is saltier and less sweet, so you might want to consider to add some sugar to it.

Ah, I’m afraid you used the wrong paste. The one you used is doubanjiang, which is fermented spicy bean paste. It will make the sauce very spicy.
The sweet bean paste should be something like this: https://amzn.to/3Eno4pn
It does not taste spicy at all, should be salty savory and slightly sweet.
In this post I also have a picture showing the brands you can find at Asian markets: https://omnivorescookbook.com/ji-dan-bing/

I think you totally can. I would use a LOT of diced mushrooms to replace the pork. A mix of rehydrated shiitake mushrooms and other types of mushrooms like button and oyster. For the toppings, you can use sliced cucumbers, lightly blanched bean sprouts, and sliced radish. The key is to use sweet bean sauce: https://amzn.to/33ZzihY (you can find cheaper ones at Asian markets)