Sounds like a great start with many species needing different kinds of attention too.
Your winters seem equal to what we have around here nowadays and therefore I can share what we do here to keep trees well overwintered.
when it gets cold in shorter periods like one week native and adapted trees will have no problems. Only trees like Gingko and Trident maples that hate cold roots will benefit from being inside a polytunnel or equal to have higher root temperatures during winter.
Pines hate to be wet, so place these under some sort of shelter in wet periods no matter how well draining the soil is. It can be a simple roof or any other arrangement keeping them from being soaked in rainy periods. Japanese red and black pines are especially prone to root rot if too wet over time. Temperature wise they love the cold, especially P. mugo.
By the way, avoid pine bark in the substrate as it breaks down quickly and ruins the overall soil structure, so it isn’t good at all for bonsai. Replace it with Pumice or lava in spring or the next time you need to repot.
It might demand moving trees around in between so they get the best possible situation. It sounds like you have covered the main aspects and concerns fine. You can bring in most trees in an open polytunnel, pop-up or stationary greenhouse where airflow is keeping fungi out, and then close it if it gets cold for some days. But open the doors as soon as it isn’t to circulate air.
Also, take care of cold winds, and winds in general if the soil is frozen. Roots can’t take up water when the soil is frozen, and winds will dry them out fast.
You should be covered if you check your trees regularly and also be sure to water if they dry out. Even when it’s cold, but not freezing, the water will evaporate from the soil, and more trees are damaged during winter from drought rather than cold (as long as the species cope with freezing). Water sparsely though avoiding trees soaking.
Most trees here stay outside but move in in cold or wet periods in a shelter. We get very wet periods and this is as damaging for the roots as freezing. Hope this helps, but be sure to ask again if there is something not clear. I will take the subject up in the next members’ Live Q&A on Thursday.