How can you design a wireless network to minimize interference from other networks?

Use the 5GHz when you can. Not all equipment will do 5GHz. Use different SSIDs for the two bands (I like to social engineer the users and call the 2GHz band "foo-slow" to encourage them to use the 5GHz band)

Read data sheets and review the radiation diagrams of the antennas you are planning to use. Focusing on the side lobes. Design your network using tools like UBNT Airline, if its on 5GHz.

My Personal opinion is it's always good to leave access point to select the best suitable channel. Meaning leaving the channel selection to automatic unless wireless network required customizations when comes to channel selection. In a environment with lots of interferance, this is really important because interferences can change time to time. So leaving it to automatic will help to find best available and optimal channel available.

Consider how many users you have. When you have a small number of users, you can go with the wider channel options on 5GHz, but if you have a lot of users (think a conference), you are far better off using the narrower channels and being able to re-use the channel at a closer distance. The smartphone apps will look for beacons, but don't tell you how much activity there is on those channels. There are apps available that will scan for activity, not just beacons.

It doesn't do you any good to have more power at the AP than at the device connecting to you. More APs at lower power and closer distances can provide better coverage when you have lots of users. Do testing to see what effect walls have on your signals. Some walls attenuate the signals, some block the signals entirely (and it can be very different on different bands)

Lowering power lets you put the APs closer to each other. Also note that the range of the signal can depend on the number of people in the room. When I setup wifi for a conference, I put the APs at ground level so that their signals are attenuated by the attendees in the room, letting me put more APs in the room then I could get away with if they were overhead. Each AP has a given amount of bandwidth that it can provide. More APs in an area lets you have more bandwidth in that area (assuming that they aren't interfering with each other)

Be careful about automatic channel configuration, as conditions change (i.e. people crowd into a room), it can trigger a cascade of reconfigurations. I prefer to have APs just bridge to the wired network, as that allows the same SSID to be used on multiple APs with transparent roaming between them as you move around (there are some clients that hang on to an AP long after they should have shifted to another one, but overall it works quite well) Not broadcasting the SSID won't stop a competent person from knowing the network is there if it's in use, it just hides it from default UIs UPnP is only suitable for home networks (and even there, I discourage it)

Privacy and interference avoidance are precisely some of the distinctive benefits that private 5G networks can offer to indoor connectivity by using shared/licensed spectrum instead of unlicensed spectrum.