1. The Mac mini form factor officially supports two external displays. One must be DP/TB3, the other must be HDMI. However, I would argue that the Mac mini is the least enticing form factor in the new line unless you have very specific requirements, and the need for two different protocols is both annoying and confusing.
2. Sidecar works just fine. This is my current solution of choice.
3. You can configure a Raspberry Pi to act as an AirPlay display. The software I attempted had a hardcoded resolution, and any attempt to change it failed.[0] You can see from that issue debugging is still a work in progress, but it's fairly tedious, and I haven't made much progress since my last reply--all of my attempts have failed to produce meaningful info. However, if you have a screen with a very common 16:9 resolution officially supported by lots of Apple devices, this will likely work quite well.
4. You can use an Apple TV as an AirPlay display target, provided your monitor is one of the supported 16:9 resolutions. 1920x1080 is a safe choice.
5. You can get an AirPlay-capable display. I haven't really looked into this, as it seems like a fairly limiting criterion. I suspect my options would be limited to projectors and overpriced TVs. I could be wrong--again, I haven't checked.
6. Some people have reported being able to get DisplayLink working.[1] I plan to try this, but I haven't gotten around to figuring out which of my adapters support DisplayLink. I'm hoping I'll be able to get this to work with the adapter plugged into my CalDigit TS3+ dock so I only need one cable.
Side note: despite the limitation, I got a CalDigit TS3+ just to dock my M1 MBA. My Intel-based MBP had four TB3 ports, which meant I could connect it to my dual-monitor KVM with three cables (2x DisplayPort + 1x USB) and still have one for power. However, even with only one DisplayPort cable, there still aren't enough ports on either the M1 MBP or M1 MBA. It works great, aside from the fact that I'm wasting a monitor.
[0]: https://github.com/FD-/RPiPlay/issues/138#issuecomment-74523...