You don't have to go ultra-expansionist. A smaller, compacter, developed empire can begin attacking before a large, diffuse, undeveloped one can. Roll the dice...

There is a strategy and a number of planets that you should minimally have but it depends on the galaxies size for instance a large 10-20 will be decent while on gigantic 30+ cause you dont want to fall behind culturally and dont worry about developing outer worlds untill much later make sure you have all your capitals and then develope them. Also generalize your planet make most small-medium sized ones research capitals so that you research weapons and armor in less then 5 turns(usually it works). Carefull not to crash your econ though.