I'd love to know if it will ever be possible to build tiny liquid-fueled rockets (with 3d-printing of the a model rocket engine being possible at some point).
but...liquid propellents are either pretty dangerous (hydrazine et al), or impossible for an amateur to work with (cryo requirements, pressures required, complex starting sequence etc).
The plumbing, extra parts, materials required, tolerances mean that what you might consider as a "classic" liquid design ends up being pretty heavy at a small scale, so you have to build a bigger engine to cope with the extra weight. So you would end up with an engine not really that analogous to what you would consider "classic" liquid designs (gas generators/tap-offs/staged combustions).
However.... Frank Malina and Jack Parsons essentially did what you're proposing in the 1930s without any new fangled 3d printing technology, so it's possible! Just don't end up like Jack Parsons.
Robert Goddard’s first rockets were basically model rocket sized.