Design for manufacturing generally means things like:
selecting components that have high availability, e.g. that SMD resistor costs 3c more, but it usually has half a million in stock for next day shipping
Reducing parts to the fewest unique values / sizes, the lower the PNP rows you need, the cheaper they are in assembly
Remove all throughole components from a design if they are not required to meet some other requirement, as they usually have to be hand soldered, slowing things down
Plan out a testing procedure, so faults can be caught at the factory if your doing a medium compexity order
look into ordering microcontrollers from the various suppliers pre-programmed, the fee is generally worth less than your time
As other people are dealing with it, how could someone put it together, or use it incorrectly, e.g. if you have connectors to some sensors etc, make it not possible to fit the wrong plug on the wrong socket, label things, silkscreen is free, idiot check your UI handling, are there undefined states for the machine, how should the machine react if a sensor or motor fails, can it detect it cheaply and reliably? kind of like smoke alarm proving circuits
Design for an enclosure from the get go, its easier to have the mechanical team give you a space and some mounting holes, than the other way around, and work with them for other constaints such as heat
Making sure things won't break off if dropped form about 1.2m (average height a box is held at by adults) this is where hot snot glue is used generally
cost reducing, if the product is only used a few hundred hours per year at reasonable temperatures, you probably do not need it to be built for a 100% duty cycle, so that 10000 hour 105C cap could be subbed in with like a 2000 hour 85C cap if it saves you enough, that 1% resistor might only need to be 5%, you might not need that crystal, the internal RC clock might be good enough, you might flip around some logic so you can use the internal pullups,
And lastly, your dealing with something that is around liquids, make sure that can't easily reach your PCB!, both so you can't hurt someone, and so accidents don't easily kill the product, adding a fluid channel is usually really cheap when already dealing with custom mouldings for products.