But making it formal can be important too. E.g. if it happened before that a supervisor was disappointed in a specific situation, or even worse frustrated and aggressive about it, then probably it's a good idea not only to have regular check-ins but also to have a written record of them.

I agree those are important aspects too. Here, I think having reminders or a certain regularity is quite important, because the "original plan" always changes and keep changing as you move forward, so having regular check-ins to discuss what everybody is expecting and by when it's good.

being available when needed, offering constructive criticism as well as motivation and recognition, being humane (do not request unreasonable achievements, or expect huge personal sacrifices, respect holidays, time to rest etc.), there's probably more but this are top three to me

Congrats!! 👏