Where to start with PCB Schematic optimization
It is not clear if you intend to go to production or just for yourself. PCBA is very expensive for small runs regardless of complexity, generally one sees prices start to fall once you hit 1ku for moderately complex embedded boards 

If this is a production board, First prototypes will be expensive but they are good place to start tinkering and eliminating components, and rethinking certain circuit sections But you should at least prove to yourself your basic design works.

There is no real step by step process it's a basically project management and cost optimization  a very complex topic .

Typically during design process , requirements are identified and a target BOM or target cost is identified and kept track of roughly. Requirements are  important, for example the input voltage range of the SMPS and Max power hugely influence the cost of components needed to roll the regulator.

At this point this may not have happened, but you may still apply a target cost, and go slashing and hacking starting with the highest per $ part and rethink some circuits.

As an example if you have many different resistors of similar values, and you determine the value isn't very critical it's better to just nudge them all to one value so only one unique part is needed instead of several. 

Going through a retroactive requirements exercise will help identify low hanging fruit/useless features/overkill in your design. 

Keep in mind it isnt very easy, it is quite possible given a set of requirements that no significant optimization can be done outside of mass volume, in that case it may be valuable to rethink certainty requirements or radically rethink your design.

Sometimes there is something silly you are doing that explodes production costs, the manufacturer could provide consultation on any cost savings they can see, e.g. an expensive process step.

As an example


if you use ten fewer unique parts we do not need to run through pick n place twice because of reel limitations
these components must be hand soldered, if you make some change it could be automatically placed.