What coolant to use, and why does the fan run so often?

Overheating is almost always caused by air in the cooling system — it has to be bled properly.

People use G12+ or their own mixes. The pinned messages suggest water/concentrate 5:1 with Water Wetter additive (freezes at −8 °C): with it the fan comes on noticeably less often.

In the woods in hot weather the fan may run constantly — that's normal.

Stark videos: bleeding the cooling system, MX 1.2 — https://youtu.be/iItNWwhdmEQ, MX — https://youtu.be/dZMHxFu8dU4