1) what kind of planet you colonize (a desert planet will never build farms )

I think that rather than directly setting the amount of agriculture, etc. you can issue an edict that grants positives or negatives to planets for following or not and and then have the planet decide if this is enough of a benefit for it to follow. For example the bonus might be donation for special civil buildings for food production, if the planet doesn't care about civil buildings but can easily produce more food then they would follow any way but if it is going to be difficult (eg. no space and have to remove other buildings) then they won't follow, on the other hand a planet that values civil buildings largely may take be willing to try harder to earn the bonus.

You could also have edicts that allow for greater amounts of pollution on planet types (or less) that would influence planets to develop more or less industry for example (or produce agriculture if this counts towards reducing pollution).

Guilds could be a fun way to theme up a slider system. The Farmers Guild tries to obtain subsidies, research grants, etc. for agriculture. They try to get planets designated as "priority farming worlds". The miners guild does likewise for mineral extraction. The Entertainers Guild for happiness granting structures, etc. As Galatic Emperor you give or withdraw your support to a guild to change how influential it is. You could order your propaganda arm to teach the worlds how important farming is. Or you could have the head of the miners guild framed if he's grown too powerful. Events would shift guild power outside your control, to try to give a feeling of guiding the fate of the galaxy, but contending with conflicting agendas.

If there happens to be a balance issue with focusing too narrowly on one aspect of development (e.g., turtilng and going all tech focused is over powered), you could use corruption as a countermeasure: As guilds become more and more powerful, corruption and bureaucracy consume more of your resources.

I really like the elegance of Brain's suggestion. Passing laws should be very blunt instruments with some kind of thematic hook. IE: A Greener Tommorrow which is about focusing the entire empire on environmental conservation (with a huge variety of effects and impacts). Then you begin mixing these things together "Nuclear Power In Every Home" which has its own impacts and after a couple law passings you have a very weird place full of mildly conflciting policies that is radically different from where you started.

Consider a system like in Rome2/Attila. You assign governors to regions and can also pass edicts in those games. Edicts are region-wide (in your example, they could affect all planets in a system, for instance). Edicts give some small bonus to one or more game features - increased happiness, increased tax income, more unit recruitment slots, cheaper production, better spies, increased scan range whatever. The possibilities are literally endless and depend mainly on your game design and balancing. You could counteract it for balance purposes as well - for instance, maybe a "Technocracy" edict gives a bonus to research on every planet in the system at the cost of reduced tax income (money is funneled into research grants).