is fine to leave always the M1 MacMini always on

Yes. Shut for long absences, say more than half day or overnight. Sleep is mostly all pain, no gain. To disable, check “Prevent…” in Energy Preferences.

Yes, sleep mode seems to create more issues than its worth. The M1 does not use much power when idle and as long as the monitor sleeps you're using minimal power consumption. It has solved a lot of problems on my M1 Mac mini.

Yes, check”Prevent computer from sleep ………..”

I usually have mine set to sleep after about 3 hours. and use a screen saver after about 15 minutes which is what runs most of the time.. You do not need to use sleep. However, I use energy save to shut down the Mac at night and auto start up in the morning which I recommend.

Leaving anything electronic on doesn't hurt it. Think about all the worlds backbone networks, your network gear ie- router, modem,etc....all these things stay on 24/7 , in the case of big internet providers, because they have to...and inside them all are the same things that are in your mini....IC's, transistors, capacitors, resistors, power supply section, etc..

FWIW, I just use "Lock Screen". Eventually the system will disable the display. Hit a key or click a mouse and enter your password or just press the TouchID button on the new Magic Keyboard and you're up and in. I don't think I have powered down in weeks.

I shut my M1 mini down each night before bed. The LG 32UN880-B display then goes into deep sleep, but is not powered off. In the morning, I power on the M1 mini, the LG display comes out of deep sleep, and the Big Sur sign-in screen is present.