Dav dimmers on thermoblock will hugely affect to temperature durring shots
Speaking as someone with a background in control systems, but not an expert in coffee machines - much less this specific Quick Mill one - I’d say it’s a bit of a can of worms… it depends on the design of the thermoblock, its control circuitry/mechanism and where exactly in there you’d add the dimmer (and which type of dimmer).
I can imagine scenarios where temperature stability and consistency could be heavily affected, but equally the opposite; a naive ’I’ll plug the dimmer in series to the heating element’ is likely to result in problems.
If you plan to use the “dimmer” to bloom the puck, I’d say that a simple switch is easier and safer - however, the issue is then that the thermoblock needs to be able to get back to brewing temperature within the blooming time (and back to ‘cool’ for a second similar shot).
Pre-infusion with hot water is a different kettle of fish (?!?) entirely - if you want a temperature that’s different than the brewing temperature, then you probably need to intervene on the thermoblock regulation: its ‘target temperature setting’, however it’s implemented, rather than the heating element. If you want to pre-infuse at brew temperature, then it’s about controlling pump pressure/timing, not the thermoblock.
I realise I’m not really answering your question… but I don’t have an answer here other than “it depends” (and I don’t know enough about both what you actually want to do, and the specifics of the machine on which you want to do it).