System Development overhaul

I've found the System Development system to be quite useful for getting new colonies off the ground and boosting up good systems, but not as living up to it's full potential.

I suggest to make System Development upgrades require a constant supply of the chosen luxury/strategic instead of a one-time cost, with lack of supply causing loss of approval or even loss of the System Development level(s). This would make maintaining control of luxury resources much more important, as well as needing to choose which systems get upgrades and which do not instead of dumping luxuries into the next best system to get it. 

As this suggestion would change luxury usage heavily, I would also suggest making the System Development levels able to be unique per-system. However, I would make them non-removable, save by holding the luxury and suffering the consequences or being invaded so that it's still an important strategic decision. Do you make the prosperous system close to enemy territory an economic powerhouse but tempting target? Or do you shore up it's defenses and make it a durable source of income? Speaking of enemies, this would have an impact on taking systems as well, due to undestroyed System Development levels possibly requiring different resources than you have. Perhaps fulfilling those desires sways their approval a little though...

Another part, I'd suggest allowing more luxuries/strategics per system development, but restrict them to their luxury/strategic tier, shifting choice from 'Upgrade now and forgo possibly better upgrades later' to 'Require more resources vs. less benefit'. It would also keep the earlier resources a little more important as they'd be required to keep higher levels of System Development in place.

One final note: I also suggest looking at the reworked luxury spawning and changes of the Steam Workshop mods Reworked Luxuries and All Luxuries in Galaxy and Material Expertise for Everyone.