#1 Don’t build more building / districts than you have pops to work on in the early game
District and building only generate resources when you have free pops to work them, but they always have upkeep costs. So if you create additional districts/buildings without having any free pops, they won’t generate any resources. But you still pay a lot of extra upkeep, which can cripple your early economy. In addition your pops will always prioritize jobs of higher level / stratum. So e.g. if you build an alloy factory your existing miners will instantly get promoted and work on your new allow factory. Which means they will leave their former mining jobs vacant and your mineral income will suffer. Therefore don’t build more building / districts than you have pops to work on. On the other hand unemployed pops also have upkeep and can lead to lower stability. So the optimal strategy is to time a new district / building with the creation of a new pop.
Note that this is mostly relevant in the early game, where your economy is still relatively small and you don’t have resources to spare. In the mid and late game your economy will be able to easily cope with some extra upkeep and it actually makes more sense to build in advance, so you can benefit from a) additional immigration pull from free jobs and b) reducing time you need to spend per planet.
#2 Planet resources don’t need to be balanced
Despite the nice balance report you see on the planet management window resources do not need to be balanced per planet. Planets resources are automatically exchanged with your global pool of resources (as shown in the top bar), so there is no benefit in balancing a planet. In addition there are significant benefits for specializing your planet, e.g. you can get a planet bonus (e.g. tech world bonus for research) and there are bonus buildings, which are more efficient on specialized planets (e.g. Planetary Supercomputer for research bonus). However, I would not recommend to specialize a planet in the early game, when you only have a few planets to work with. So in summary you should focus on balancing your overall economy and aim to specialize a planet in the later game.
#3 Always maintain a strong fleet
Even if you do not plan to go to war immediately you always want to maintain a strong fleet. Firstly, a strong fleet will deter other empires from attacking you. Secondly, you will often find some space bugs in your territory, which might block your expansion and need to be exterminated. And finally, it helps to make sure your economy is build in a way that it can always support a strong fleet when you need it.
#4 Don’t expand your empire too quickly
Expanding your territory is important to get more resources and planets to colonize. But there is a prize to pay for expanding too quickly, which will not be apparent at first. Your empire has a maximum “allowed” size called “administrative capacity” and if you expand further your costs for research, traditions, edicts and leaders will increase. E.g. your research and the time to get new traditions will slow down and you risk to fall behind unless you take actions to compensate. So don’t expand too quickly unless you have the economy to support it and are ready to make sure to generate additional research and unity for compensating the extra costs.
But – if you know how to compensate for the penalties going over the admin cap is actually pretty normal. As a rule of thumb going up to 2 x your admin cap is usually not a big deal and some players will even completely ignore it.
#5 Always keep an eye on your unity
The most powerful boosts for your empire are the traditions and ascension perks. The traditions will improve many aspects of your empire from military power to prosperity, while the ascension perks allow you to specialize your empire or get powerful new capabilities like building mega-structures. In general you want to make sure to complete all traditions and get all ascension perks before the end game crisis hits at around 2450 at the very latest. To do that keep an eye on your unity income and especially on the time it takes to complete a new tradition (which you can see on the top of the traditions window). On average you want to stay between 60 – 70 month per tradition in order to finish all traditions between 2400 – 2450. Note that they are lots of ways to generate additional unity income, e.g. with unity buildings, ruler skills and edicts.