How can I discourage mycelium and other fungi growth in my plant containers? And yes, is it even a good idea?
It's not something you should worry about - there's no way of getting rid of mycelium without using completely sterile soil. Even if you did that, the air is full of fungal spores and they may arrive anyway. All you can do is not overwater and ensure good drainage by not using pots without holes, etc,to discourage invasion by less friendly forms of fungi. In some cases, there is a beneficial or even symbiotic relationship between certain fungal mycelium and plant roots.
Under 'other fungi growth', you might be including fungal growths and infections which have aerial forms, like the mildews, black spot, rust, some varieties of phytopthera infection, but these do not produce mycelium in the soil, so I'm not sure whether you're including those in your question or not. Most are encouraged by warm, humid or damp, still air and insufficient air flow round plants; powdery mildew is often encouraged by dryness at the roots.
This is WHY potting soil is sterilized! Did you try to sterilize any of the soil you made? Shoot, there are some fungal and bacteria colonies you should embrace. Such as Mycorrhize! Especially if you are dealing with unknown garden soils/amendments in pots!!
You should have FANS going 24/7 for indoor plants especially those in crappy garden soil that hasn't been sterilized. This is one of the best preventative measures for fungus. Knowledgeable pruning and bio fungicides applied early on susceptible plants are two other measures to understand. Fungicides are primarily a preventative sort of a 'raincoat' for plants. There are fungicides for ornamental plants and a few for vegetables. Milk and water seems to be very effective for powdery mildew. Any other fungus needs preventative measures such as aeration pruning, fans, 'raincoats', not allowing water to splash up on plants (containing fungal spores galore), shielding from rain (get ready for powdery mildew, it is the lack of 'free water' or physical rain, water on the leaves, humidity/dryness such as under eaves of homes where rain is unable to reach that promotes powdery mildew as well as a plant's natural susceptibility).
I also wondered about this at some point, because a potted plant I had died of damping off.
I've since incorporated cinnamon into a potting mix I used into an homemade self-watering pot. I thought some antifungal was a good idea in a constantly moist soil.
Maybe my reasoning was wrong, and there is no increased chance of fungus developing in my self-watering pots, but the facts are that I've had no problem whatsoever in the last 6 months, and cinnamon might have been the key. I also read chamomile tea could be used similarly to avoid damping off.