In case you are asking this for EMC purposes, the return path isn't as much a concern as sharp edges from the oscillator/PWM/LED driver etc causing radiated emissions. You can still get crosstalk and similar at modest frequencies, but that can be avoided by common PCB layout best practices. It all depends on what EMC requirements you have on the product.

In this project, there are no formal regulatory or compliance requirements. However, I want to design the board the "right" way—not just to make it work properly, of course.

I'm trying to determine whether I should take all the considerations I've described into account or if this is a relatively simple board, allowing me to disregard some of them. For example, could the board be designed with just two layers? After all, a two-layer board isn’t necessarily a bad choice.

Eyal78‭ You can't really design anything without requirements. Normal commercial EMC requirements are pretty lax. It gets harder if you have anything wireless in the product. And then various areas of application have their own EMC requirements: industrial/automotive/medtech etc.

Nowadays I would only use 2 layer boards for really basic stuff like adapter boards for connectors/mechanics. Or in case you have to use through-hole components because of power electronics. There's no cost advantage of 2 layer boards since a decade or two back, so why use them when you can get something better for the same price.