Is it safe to use an Intel Core i7-12700 at 65w with a Gigabyte B660m DSH3? I know that I should get one with VRM heatsinks but I plan to downgrade to an Intel Core i5-12400f. I won't operate the i7 at 180w.
It will be fine it will just operate the 12700k at whatever W the board supports, you don't have to lock it down to 65W and lose a whole bunch of performance the board will limit it to whatever it can support.
Locking it to stock settings is good for heat and power use though.
If you can put some airflow above the VRMs it will be even better.