Hi @Epic13,
I’m no expert, but judging from your excellent photo, I’d tentatively suggest that your mixing valve is in the wrong place in the circuit, and that this may be part of the problem. At the risk of over-exposing the “standard arrangement” sketch that I produced (with @Robert’s invaluable help), I would expect something like:
It looks like your arrangement has your bathroom/basement UFH feeds F/G coming off before the splitter valve B (which you call mixing valve), and your temperature sensor C is downstream of the mixer rather than upstream.
I would have expected a splitter 3-way valve (if upstream, mixer valve if downstream) on the buffer exit, after the temperature sensor, with flow split 3 ways (1st floor, 2nd floor, bathroom/basement) after (each with their own pumps).
As it is, your mixer valve appears to simply circulate unheated water around your radiator circuit.
I’m happy to be re-educated by the experts on the forum @glyn.hudson?, but it looks to me like you ought to consult another installer…
Sarah