A recent post I saw somewhere re a Glowworm "low water pressure" alarm on boiler/pump start up got me thinking about your very first post paragraph re S.53 & S.54. The glowworm alarm (and start inhibit?) is triggered by a pressure sensor which detects the small rise in pressure when the circ pump starts, so obviously if the pump doesn't start or is fouled with sludge etc then this alarm annunciates, the glowworm sensor apparently can get blocked very easily and a temporary fix was to increase the filling pressure and might lead one to falsely believe that this sensor was only used only as a method of detecting sufficient water in the boiler.
Valliant informed you that S.53 is triggered by the 30C flow/return deltaT but I think its triggered by the above (borne out by changing the pump) and S.54 is the deltaT alarm, however if this is correct then the boiler would have to fire up, perhaps on minimum firing?, to enable the high deltaT to develop between the flow/return or else S.54 isn't configured quite correctly. (Or else the boiler just waits and waits to see if S.53 clears).
I also see that you can run the boiler on return temperature by changing d.17, even though you seem to be getting very satisfactory performance now it might be interesting to try this control mode sometime to see its effect on the target (flow) temperature and of course condensing benefit, it is primarily suggested for UFH.
Also may be worth checking that d.18 (pump overrun) is on its default setting 1 ( comfort or continuous running).
Strange, but in all those interesting blog posts I havn't seen it mentioned anywhere that the flow trip is SP+5C.