When draining water elsewhere from the system, it never has brown gunge in
> it, it’s nice and clear. The boiler filter has very little magnetite in it,
> at its annual service.
It sounds as if your system is unnecessarily well looked after. My system is basically 40+ years old, and has only been drained when necessary, to replace parts. Second replacement boiler, one complete replacement 3-port (thought several heads), one replacement pump, and so drained/partially drained maybe three times. The thing is, I have always ensured, there is inhibitor in the system.
I have never cleaned the F&E tank, but a few years ago I did swap out the galv steel one, for a modern plastic one, simply to avoid future problems of it rusting, and leaking.