Galvanic DC-DC isolation without inverting/rectifying
DC-DC galvanic isolation possible without inverting/transforming/rectifying?

For power transfer with isolation we generally use magnetic conversion, by that I mean the electric energy is converted to magnetic energy and back to electrical energy. For that we need a varying magnetic field meaning an AC input signal is needed and rectification is also needed. You cannot transfer energy with a static magnetic field.

The only solution I can think of is optical so a (LED) lamp shining it's light on a solar panel. That will be big, inefficient and somewhat expensive perhaps.

So possible: yes

But practical: no

I agree with Finbarr's comment that you very likely do not need galvanic isolation. What you need to do is take care that you use proper grounding that does not mean a stick in the earth but following good practices like a star ground setup.

Also, current travels in loops. Make sure the loops that the disturbing currents follow are not shared with the loops that the currents in the sensitive circuits follow. Using a start grounding scheme helps with this. To separate the supplies use voltage regulators and plenty of decoupling.

A general tip: try to avoid "fixing" a certain problem (separation of circuits) by introducing something complex and costly (galvanic isolation) without fully understanding if the problem actually is a problem and what the potential solutions are to fix that problem. Only if that proves that there is no other solution that those drastic solutions should you consider those drastic solutions.