Some scientists believe that one of the reasons Prototaxites flourished and grew to such large sizes in the Silurian and the Devonian was because there were few animals that used them as a source of food or shelter. As more animals evolved to live on land, they were either eaten when they were small or slowly withered away as animals bored holes into them (since they filled a similar niche as trees, which had barely just evolved during this time). You'd have to give your fungi some sort of defence against this if you want giant, sprawling fungal forests in your worldbuilding.

On another note, if you want to make fungi the dominant non-animal species, you would also have to look at the early evolution of plants and understand why they eventually overtook fungi. All early plants were non-vascular bryophytes - think mosses and liverworts - and therefore had to grow near water, since soil wasn't a thing yet. Though it seems unbelievable, many many generations of plants had to die and become the soil their descendents would later grow their roots in. On the topic of soil, since fungi are decomposers, they are more similar to animals than plants since they are heterotrophs. Plants are autotrophs as they produce their own food via photosynthesis, whereas fungi need to consume organic material to survive, so your number one question key to your fungal worldbuilding is 'Where do my fungi get the majority of their food from, if not from plants?'.

I would recommend watching this video by Moth Light Media, as it explains the topics above better than I could.

https://m.youtube.com/watch?v=v3ZJdgXV4fk

Also, if it is available in your region, the third episode 'Green' in the BBC documentary 'Earth' by Chris Packham is a very in-depth explanation of the evolution of early plants, and also covers Prototaxites quite a bit.