Doesn't even need to be attached to the heat source, just thermally coupled. Could be achieved for example via a short, fat, insulated, shared ground plane or s

Doesn't even need to be attached to the heat source, just thermally coupled. Could be achieved for example via a short, fat, insulated, shared ground plane or something. The compensation might be needed, and the internal thermal sensor also needs tuning in any case if you read the specs, but it's close enough that you could use it out of the box and only be out by a few degrees. Chip temperature with regular sleeps isn't going to be a significant factor. There is enough code space on the ATtiny85 that you could actually build a model that takes measured temperature T at time t (T_t), the goal temperature of G (G_t) and produce a predictive model for M(T_t, G_t) = T_{t+n}. You could then do something a little smarter than a PID controller that takes in some profile considerations, i.e. slow (low noise, low RPM, likely for normal usage), fast (high noise, high RPM in bursts, likely for gaming).